"Where are the tall schools?"
Translation:Hol vannak a magas iskolák?
Other than the typo, "iskolákat", you have put the subject in the accusative. It should be nominative. "Iskolák". It is the subject of the sentence, not the object.
You do not decline (modify) the adjective when it come directly before the noun. You use the plural form only when it is a predicate adjective separated from the plural noun it modifies.