Could it also be "the boys are students" ?
That would be the other way around. "A fiúk tanulók" but if you mean just the word "student" instead of "pupil" then yes it should be accepted.
Why is it not: A diakok Fiukat?I thought the obj. has a 't' attached
It is not the object of the sentence.
Why is this not correct?
As someone said, there is no object in the sentence. We could argue whether there is even a verb in the sentence - but there surely isn't a transitive one.
The boys are students...
No. The would be "A fiúk diákok "