NHF – new heroes for Heroes V 3.1.

Couldn't you set a specific skill level for a specific hero, regardless of the development level of the corresponding school, so that they could then cast any spell they wanted? Besides, there isn't even earth magic.

The skill level is defined for creatures; the hero casts a spell based on the formula corresponding to their skill level. However, if you give them an "extra expert" level in a school of magic (which is possible), they will be able to take 4 perks instead of three, meaning one of the perks won't be displayed in the hero's window (but it will still work). This looks a bit awkward, and besides, it will strengthen not just one spell, but all spells that have a formula for the "extra expert" level.