I lean towards it having to be number of build based because of how much longer it would take to go from Scholar to Fighter than the reverse due to having to drop spell slots singleton.
If the Teacher ability allowed one to respend all of the build in a given Skill, even if (and likely when) that resulted in respending some build back into that Ability, it'd balance out, I feel.
<Magic School> 1 through Formal is 10 'Teacher Card' uses.
<Martial Skills> are subjective, but potentially higher requirement, given they all (currently) tie back to how many Weapon Proficiencies a character has.
Requiring a top-down approach would allow it, I feel, without becoming too restricting.
IE: Fighter wants to go Scholar? Okay, well, first get teacher cards for each Stun Limb, Parry, Slay, Eviscerate, Riposte, etc. before you're able to 'Teacher'-away a single Weapon Proficiency.
Scholar wants to go Rogue? First get rid of your Formal Magic, then your 9th levels, your 8th, etc. down the line.
After you 'zero out' your abilities, start collecting Teaching for your new abilities. Don't want to deal with that much work? Deal with different work in looking for a Spirit Forge scroll and catalyst.
Want to go through it slowly?
Fighter (as above) loses all their disarms and Stun Limbs one event. Next event, they lose their Slays and Parries. They slowly reinvest that build into Scholarly abilities -- starting from the ground up -- until they get down to Weapon Proficiency, losing the last vestige of stabby-stabby they had.
Scholar (as above) drops their Formal magic and 9th level spells, this results in a teardrop-to-8, putting that build into Backstabs and (now) Create Trap or Alchemy.