The only thing a magi needs an apprentice to have to be useful is Intelligence + Magic Theory. An apprentice doesn't even need to read for that. Have a mundane teacher read some Magic Theory books. Even if the apprentice is Gifted, he'll still learn 10+ xp a season from that teacher and in less than a year he will have Magic Theory 3.
There are a number of good reasons that magi schools don't happen. First is that magi don't want their apprentices to become powerful! Young magi are hot headed and do dumb things that upset older magi. The more powerful they are, the more likely this will happen. It's the same reason you don't teach them formulaic spells until the last few years of their apprenticeship. Would YOU trust a teenager with the power to control minds or move heavy objects with magic? Of course not! This same reasoning limits the teaching of Gentle Gift. A GG magi can amass mundane power easily and doesn't need the Order for much. Normal magi need the covenant structure of the Order because the Gift causes mundane society to reject them. Teaching all new apprentices the Gentle Gift would radically change the Order's new generation and cause a serious schism between old and new magi, and the old magi like the status quo. Powerful young magi are just going to get uppity.
Second, if a master wants to control his apprentice, he will want to start young. Handing him over to a school for 5 years just gives someone else control over his apprentice, which is unacceptable. You might as well hand over your secrets to the training school. If the apprentice turns out badly, you as a master can kill him and that's the end. But if he went to a school that apprentice now has friends who will become magi eventually and might seek revenge. An isolated apprentice is an apprentice you can control. OTOH, an apprentice with Gentle Gift isn't isolated with only his master to turn to. That would make him harder to control.
Third, magi can see what happens at universities in Europe, and they're sure to not like it. Most towns with universities have endless troubles with concentrations of young men free of parental influence. Now imagine those same circumstances, only they can do MAGIC. Gangs of youths are trouble no matter what the century.