Does anyone have any inexpensive evening activities for high school students in New York City?

If you're not talking about the cold weather season, how about a ride on the Staten Island Ferry. It's free! It passes thru the NY harbor and past the Statue of Liberty. I would think it would be a fun jaunt for high schoolers.
Another suggestion: If you plan far enough ahead, perhaps you can get tickets for a taping of the David Letterman Show. That's also free.
As for things that cost money, what you you call "inexpensive"?

Go to the top of the Empire State building- very cool at night (even for adults).

Oh yeah, I almost forgot, Conan O'Brian show is free and and if you call approx. 3-wks ahead you should be able to get tickets w/out any trouble. Again, fun for adults as well.

You have to be 18 or older to get tickets to David or Conan ... and I hear it takes a year's advance request for Letterman tickets.

Are these students who live in NYC already, or visitors?