"The lazy teacher is writing onto the window."
Translation:A lusta tanár az ablakra ír.
This is a little different than the sentences I usually moan about, because it doesn't sound quite right, but it seems like it might at least be grammatically correct.
On the other hand, Wiktionary defines "onto" as "on top of", which isn't the case for the window, unless the teacher took the window out of its socket and started writing on the top edge of the pane. (I'm guessing not, since the teacher is lazy) At any rate, just "on the window" would suffice.