Not much flex necessary to get a 2 with Affinity with Parma Magica, no, if the last season of apprenticeship was spent instructing it.
Basic SQ for Teaching is Com + Teaching +3.
Com -2 is mildly extraordinary in the wrong direction. The previous 14 seasons of Teaching a magus's very first apprentice is itself worth 28 XP from exposure, so the instructor easily has a Teaching of 2 (even if they divert almost half that XP to improving the language of instruction), and a Specialty applicable to the student he got 100% of his teaching experience from instructing is reasonable ("one-to-one" is canonical in Covenants, on p.43). Which means we're now at (-2+2+1+3=) 4, and then there's a +6 for one-on-one instruction that should apply to any apprenticeship-qualified instruction season. So that's a SQ of 10 from a teacher who started the apprenticeship with zero Teaching, awful Com, and no applicable Virtues.
A 2 without Affinity is slightly more flex, but not that much. Getting a total of 6 from Com, Teaching, Teaching specialty, Virtues, and lab specialization isn't all that hard (the basic formula +3 and the one-on-one instruction +6 bringing that 6 up to SQ 15).
A 3, with Affinity, needs an SQ of 20. [EDIT: Well, 19, really, and round up, right?] Now you're talking a teacher with a Com, Teaching, specialty, Virtue, and lab bonuses that combine to +11 [+10], which will indicate some degree of actually notably good instructor for a magus. But not all that much. If you spent a Virtue on Skilled Parens, I would actually consider it explained.
A 3 without Affinity, now you're talking someone with PC levels of task-specialization, to get 21 out of Com, Teaching, specialty, Virtues, and lab.
But let's consider such a PC specialist.
Affinity with Teaching minor Virtue, and 60 seasons of exposure to Teaching (three previous apprentices, 14 previous seasons with this apprentice, and one other season along the way somewhere) works out to 180 XP in Teaching for an 8. "One-to-one" specialty is another +1, bringing us to 9. Com 5 (improved with ritual magic in play), the Good Teacher minor Virtue (+5), and a Teaching-optimized lab (Gallery and Palatial for +3) get us to 22, we add the standard +9, and we have a Source Quality of 31, able to teach an average student Parma Magica 3 in one season of instruction.
Add Puissant Teaching to push to 33, and our student with an Affinity in Parma Magica can go from 0 to 50 XP, for a score of 4, in one season, under this instructor.