And now to make it complicated:
Sehen Sie den Lehrer? = Do you see the teacher?
"Sie" in German can be three different things in English: "They" (plural), "She" (singular), or the polite "You" which you adress people with that are above you in hierarchy or older.
She always needs singular ending. This is not the case here. The two other forms need a plural ending, so they are an option here. But since the polite form clearly doesn't make sense it must be the third person plural they.