It is my opinion that almost any mathematician, even beginners, will agree that as the power does not apply on the minus sign (or, if you prefer, on -1) but only on the immediate expression it is upon.
Now, it could be that your teacher considers as asingle expression (which, in fact, it is: the additive inverse of ), but then we have a problem: how could we express the mathematical expression "the additive inverse of (3 raised to the second power)"??
Should we write the minus sign further away from 3: ?
These are question I'd ask, very respectfully, my teacher if I were you.
In general, I'd say you're right.