just wondering how am i supposed to know which spell schools i can use as necromancer/any other class really, sometimes it says i cant write magic on some schools but i cant find how you find out which schools you cant use

thanks in advance

Check out Planet Baldur's Gate. If you're referring to BG2, there's a table that lists the schools (with opposing schools) Here. Spell lists are listed Here.

Hope that helps.

General mages can use spells from any school.

Specialist mages are forbidden at least 1 school ie-a Necromancer can't use Illusion spells.

The table for forbidden schools for specialist mages are found in the Tables section (the last section) of both the BG1 & BG2 manuals.