There's a contradiction in the rules, and they use a wrong term (prohibited instead of opposed).
A wizard can prepare a cantrip from a prohibited school, but it uses up two of his available slots (see below).
and on the same page:
A wizard begins play with a spellbook containing all 0-level wizard spells (except those from his prohibited schools, if any; see Arcane Schools)
If so, do you have to get the opposed school cantrips later on from scrolls and scribe them? Or do you actually get all of them, and that's just a leftover from 3.5?
I don't think it's a contradiction so much as the master who taught you thought that you wouldn't have much use learning a cantrip that would take up two slots so he never gave you a copy to copy out of his book.
Insert dialogue from crabby mentor here.
You can probably pay a universalist wizard to copy it out of his book, or just find another specialist with a different forbidden school and you copy the missing spells out of each other's books.