So, i suggested before that you should get more access to spells in secondary schools, to buff them as secondary schools, so they can compete with the tried and true meta. Allow me to further elaborate into a few ideas for post-classroom tutelage.
The first one is a more obvious one, the trees! They already teach you how to craft cards of your school's spells, and they're more ancient than basically anything else in their vicinity, so it feels like a logical next step that they could teach you some spells after even your tutor. I mean, Niles actually teaches you the balance blades and traps already! Good suggestions include: A small handful of their quest spells (not all of them, to keep each school unique and not completely replaceable by another), or of course, fan-requested spells.
The second I mentioned before, so I'll keep it brief, but Pigswick. Every school is taught there, but in a different way, so they could give access to some very interesting spells. Among the ones that could be added are permanent versions/twists on mutations, such as the version of Storm Cat that Stormclaw gives you, or the Life version of sandstorm one of the Kookaburra pets gives you.
The third is also an interesting one. I've heard of a land of Darkmoor, and it appears to be an undead haven from the little I've heard. My idea there, is perhaps spirits of past tutors for all 7 schools teach in a new part of that world. Perhaps spells there utilize shadow or somesuch. I don't know too much about the deep intricacies of the lore, to be fair. So that could be impossible. Alternatively, spirits of major bosses defeated could be found here, and they could teach spells based on bosses, but likely more powerful than the TCs for each of them. Lord Nightshade, for example, could be the Death instructor for this new branch, and give spells of like, Grubb, and Rattlebones. They don't have to be world bosses, in fact, some point could be made for them all being major bosses, but avoiding world bosses for the most part. the 3 major bosses of Krokotopia (including Krokopatra, breaking this rule already) could be the Elemental instructors, some other 3 bosses could be the spirit classes, and balance is already a wild card, so i dunno who would teach it in this case, there aren't many balance bosses.
The fourth suggestion I can think of, and the weakest is just expansions to the existing side tutors. Yknow, the ones in the Secret Stores, that you only visit when they teach things within your school? Yeah, what if they were more relevant later?
The fifth suggestion, is to have Ambrose himself, or even Gamma, teach a few spells. I mean, both of them have more magical knowledge alone than the rest of the school combined, and its clear you are his favorite student rather early on, given how he trusts you, and you alone (alongside the millions of other player characters) to defeat multiple ancient evils over your career.
The sixth, and easiest, to finish us off, is just to give our seven teachers more to teach I mean, at this point, their curriculum ends not even halfway through the game. Their ending point was great back in the days when Celestia was new, and you could only complete one and a half to two schools if you focused really hard into them, and skipped out on Celestial schools, but nowadays it just isn't enough.
Regardless of which, if any, route is taken here, it feels like each of the secondary schools could use a nice boost to make them relevant again. If multiple of these are added, it could give each primary and secondary school some nice versatility and unpredictability within the realm of PvP, leading to a healthier meta. Perhaps to coincide with this, a free buyback weekend could happen. Or every existing character past level 42 (when your secondary school normally stops providing new spells) could get a coupon for one free buyback.
