My screenwriting teacher for a writing assignment asked what turns us on? ?

I don’t know.  How are you interpreting it?  The question may broadly mean, what motivates, inspires or desires and vice versa?  Finding the motivation in the human character answers a lot of questions and explains behaviour.

I can walk up to a great looking blonde (turn on) guy and find he reeks of smoke (turn off) 

They are purposely leaving this up to your interpretation.  But I think you are taking this too sexually

They don't want sexual preferences,  they want to know what you like to watch on tv.  Do you like science fiction,  and why. Or adventure,  or romance,  or historical.  What do you refuse to watch? Why?

There's a range of meanings for that phrase. Let's assume your instructor is neither incompetent nor a sexual predator.

In that case, s/he probably means "What gets you enthusiastic and excited?" and "what is so negative you cannot summon any interest?"

So your answer for what turns you on can be, say, teaching illiterate adults to read or your young nephew's wonder at everyday things you take for granted. Turn offs might be people who complain about things that aren't important, or greedy corporations.