Translation:The buses are not in front, but behind.
Use valami előtt if you want to say "in front of something." A tanár az iskola előtt van, the teacher is in front of the school.
Use elöl if you mean "in the front" or "up front" or "in front, generally". A vezető elöl van, the leader is in front.
Likewise, valami mögött means "behind something" but hátul means "at the back" or "behind, generally."
"In back" is correct. The word "behind" cannot stand alone, just like "mögött" cannot stand alone. Behind the table = Az asztal mögött Behind the chair = A szék mögött Furthermore, the word "behind" is a preposition. You cannot end a sentence with a preposition. Therefore, it is grammatically incorrect.
My answer: The buses are not in front but are in back. This was marked incorrect. But the Magyar-Angol Szotar gives this definition of hátul: "at the back, in the rear." To say something is in the back is perfectly good English usage meaning, in this case, the buses are at the back side of the building, or whatever the referent is.