What should I wear to go see a play?

That will be fine. What your teacher means is no t-shirt, cut offs and flip-flops.

And try to pick jeans with no rips.

If you have a red tie, wear it just for fun.

If you have slacks, those would be perfect.

As long as the jeans look new and your shoes are super-clean, it should be fine. I second the idea that @Jeruba suggested about the red tie. Khaki pants would be good too.

Jeans might be OK, but slacks/pants would be better. It sort of depends on where you live. Do other people (adults) wear jeans to nice events? Whatever you wear, nice shoes will help you look dressed up and put-together. And remember to brush your hair! (LOL)