What the heck is a tall school?
Why is "Hol vannak a magas eskolákat?" considered incorrect?
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.
When do we use magas iskolák and when magasak iskolák?
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.
So could I say
Hol vannak a iskolák, azok magasak?
There seems a tendency to use the adjective "tall" in unlikely situations. One does not usually think of schools as tall or short. I have seen this same usage with other nouns as well.
And it makes no sense in English. Do you suppose they mean "high schools" as in "secondary schools?"