In general, it should be exposure.
For it to be practice, the teacher would need to receive "immediate feedback" about how well he's doing. Which in general is not true when teaching (from my own experience), unless you are teaching under a third party's supervision:
Third, being forced to practice a trade or craft in an environment with immediate feedback as to how well you are doing has a Source Quality of five. An example would be someone forced to help on a ship.
Besides, practice is a full-time activity. So while practicing teaching, you can't also be teaching someone.
But just as someone living in a city while working can claim on of his free seasons to claim a SQ 5 on Area Lore for the city, I think it would be reasonable for a full time teacher to claim SQ 5 if he uses his free seasons to practice his teaching, perhaps even SQ 7 (since working in a city as a messenger merits 7, and I'd say working in a school or university as a teacher is not that far apart).
Still, notice that you are not working and gaining practice XP. You are working your standard 2 required seasons (and gaining exposure for that), and claiming practice XP on your free seasons.
I think you could, in principle, gain Practice xp in Teaching while Teaching if, while you taught, someone more experienced "assisted" you in the task and provided constant feedback (gaining only exposure xp in the season).
Having a mundane do it while you teach your apprentice would feel ... a bit silly, but not entirely out of the question. Having your familiar do it would actually feel right. Of course, you'd need a familiar well-versed in teaching!
A simple rule of thumb is that if the season brings other value, you gain only exposure. The exception is stories. If you practice, you are not productive.
What you suggest sounds like training. A teacher can train (AFAIU) a student in teaching while they teach (and thus earn their living). The trainee would in some sense be engaged as teacher, but it is the teacher-trainer who mechanically generate the teaching.
No, it's different (and it's not me suggesting, it's the corebook).
With training, the trainee gets no productive work done - the trainer does, though; with "practice with feedback" everyone gets work done (e.g. "being forced to help on a ship" does produce something of value). On the other hand, being trained has the potential to be significantly better for the trainee, since he gets (trainer's Score + 3) xp rather than 5xp.
In this case, training a magus in teaching means someone else does the teaching, and the magus listens and learns (of course, if that someone else is a mundane, there's no way Arts can get taught). With practice, the magus does the teaching, and someone else gives him feedback. It's a bit hard to see how that "someone else" in this particular case could get something useful done ... but it's possible in principle I guess. Maybe a magus teaching the Arts to an apprentice who was a Magister in Artibus could claim practice xp, if the apprentice gives him "feedback" about teaching? Seems a bit forced, but...
That's a very odd interpretation of RAW. It says explicitly
forced to practice, and
environment with immediate feedback,
A supervisor is not an environment, and immediate feedback is quite a strain on the supervisor, who can hardly expect to have a full season's worth of production on top of supervising duties. And most certainly, this is not a forced situation. It is simply not possible artificially to recreate the feedback of learning the hard way on board a ship at sea.
A character can try to deliberately find out more about a subject, by her own efforts.
If someone else is helping you you are not on your own anymore. And if someone is making remarks that essentially amount to "one-on-one training where the master shows the trainee what to do" that's training, as defined in the corebook.
Furthermore, still on Practice, pg. 164, end of the paragraph:
(...) it is a full-time activity.
That's game lingo for "you can't really do anything else on a season where you practice something". So no, practice does not get anything else done.
What about
being forced to practice a trade or craft in an environment with immediate feedback (...) An example
would be someone forced to help on a ship.
If you tie a knot wrong, mess up with the sails or something like that, things will go wrong badly with the ship. You don't need a person to tell you the ship is sinking (or, to be practical, that the sail is loose). It's immediate environmental feedback.
OTOH, let's not forget that most people don't have genuinely free seasons. If I am practicing Profession: Sailor in a ship on my free seasons, I'm very likely also working as a sailor on my working seasons. The way this looks from the outside I spend the whole year working as a sailor, but while my colleagues are resting or carousing I'm doing a bit of overtime because "I really want to know how to tie this knot". It's not like I woke up some day and decided to find a ship because I had nothing to do, and just boarded it and no one paid any attention to me.
That's also why, by the way, I'm not being trained by another sailor (which is more useful (to me) than practicing). We both work 2 seasons, since we need to eat. We need to work at the same time, so I can't get trained during the day and work as a sailor a night. He has no obligation to teach me anything (I'm not his apprentice), so he uses his free seasons to do whatever he wants. I use mine to practice.
How that looks like for a full time teacher?
You work every day from morning to afternoon. On evening, instead of going home, you double check your teaching notes in your office (practice xp for you). If you get lucky the dean is also a workaholic, so he might show you how he organizes his teaching notes (teaching for you/exposure for him), or you just make small talk over a cup of coffee and compare how the other preps his notes (practice for both, but this isn't quite immediate feedback, so each gain 4xp). Note that the dean (or you) prepping his notes is not actually teaching unless a student is getting taught, so he can't train you on prepping notes because that's not work for which he is earning his living (we have explicetly framed this example as preparing notes on his free time).
Training can be done while working if you are using the skill which you are training the other person in. There is no additional work accomplished by the person who is being taught. I've always understood this to mean that the work you lose by taking time to train someone is balanced by the work they produce under your tutelage, and from a bookkeeping perspective the work accomplished is the same as if you had done it all yourself.
This may be the case, but then we should be clear about how nominal seasons work. The teacher has a work season at daytime over half a year, and a free evening season stretching over the same half year. He gains exposure in the daytime season and practice in the evening season.
More likely they will use the evening seasons to practice music or carouse than teaching, but that's quite a different matter.
I am certainly of the opinion that experience is underrated (and books overrated) in Ars Magica, but the solution is not to jack up xp and build a higher power house. Encouraging all PCs to be monomaniacal workaholics is not going to make better stories.
Ah, yes, I was not suggesting that every teacher (or any other worker, really) uses his evenings to practice their trade. I expect most of them to use them to spend time with family and friends. As if they were, you know, normal people.
So you are saying that practicing a Living Language means talking to yourself all season long?
No, that's game lingo for "On a season of practice, you can carry out no meaningful activities other than those related to practicing -- but of course those activities may produce both relatively valuable lessons and some useful, concrete outcome. Case at hand: if you are forced to help on a ship, it means that the ship is at least a little better off through your efforts - or you would not be helping.
Note that under training, it says that
A character must have a minimum score of two in an Ability before she can serve as a master, and at that level simply yelling at the apprentice when he does something wrong is equally helpful.
So, clearly, yelling at the apprentice when he does something wrong is not training, it's something else ... that generates 5xp/season. Yes, practice.
No, that does not follow, and is most definitely not clear. In fact, the part you bolded makes little sense, since it tells us what it is equally helpful as. But in no way does it imply anything about practice, nor does it say that yelling makes it not-training.
What it's saying is that training, from such low-skilled trainer, is equally helpful as "simply yelling at the apprentice when he does something wrong". This means that simply yelling at the apprentice when he does something wrong is something other than training him. If you say "Adam is no taller than Bert's brother" you are clearly implying that Adam and Bert's brother are different people, unless you are discussing logic.
So, what type of Advancement is this one where the apprentice is clearly getting:
a) exactly 5xp/season while
b) being forced to help in an environment where someone provides immediate feedback (in the form of yelling) when he does something wrong?
The only possible answer is Practice.
If this isn't obvious to you, there's not much more I can say - so I'll just drop out of this
No. I'm saying that if you are doing the heavy lifting, that's practicing. If you are on the receiving end and another person is being burdened by you, you are being taught or trained.
There's a difference between immersion language learning were you talk to a lot of people and try to make your way through it and having your errors systematically pointed out to you by a single person.
Another example: sparring with someone is as much practice as is practicing your strikes alone against a tree. In the end you are trying to figure things on your own on both cases.
Sparring with someone better than you who is explicitly trying to inculcate knowledge into your head, on the other hand, is either training or teaching.