Also, time. Given state of the world, do you know how many beastman raids, greenskin attacks, skaven sabotages, Chaos incursions, dreams-that-turn-you-into-a-daemon, random mutations, etc. can happen in the general period of time it takes for IRL traditional schooling to take place?
Some students need to leave back home to the farm because half of the herd turned into rutting beastmen that slaughtered/stole the other half of the herd after they were done with their emergence orgy.
38 year old Timothy who was 1 year away from graduation was walking back to the school at night when a beam of green light from Morrslieb shined down on his bald head and he sprouted a horn through his brain made up out a rapidly growing spinal column which turned him into a bone tree.
And so on.
It takes time to become fully educated, barring people who are just naturally geniuses, folks have to study hard - for years - just to become relatively competent. Setting aside the time it takes to properly instruct the fully uneducated on matters of reading and writing before all of the other instruction.