"Not the students are in front of the new school, but the teachers."
Translation:Nem a diákok vannak az új iskola előtt, hanem a tanárok.
I'll just go into details then. :)
Diákok needs to be in the focus. The focus is the main emphasis of the sentence, if it's not the verb itself, it's always followed by the verb. (Note that the focus might be empty, so the word in front of the verb is not neessarily the focus.)
Diákok is negated here, which usually means that it's emphasised. So even if we didn't have the second part, we'd still need it in the focus.
Also, the sentence is used for contrast. Nem a diákok, hanem a tanárok. Such construction usually has diákok in front of the verb. [Previously I said that it required diákok to be in the focus, but I was wrong. I'm editing accordingly. Apologies.]
It is not good.
The part "nem a diákok" is OK, as nem should be placed before a diákok, this is the part we negate.
The position right in front of the verb is the most important in the sentence, and now this role should go to "nem a diákok". Therefore, "Nem a diákok vannak..."