Not paying tax on summer employment

After looking at the form and the IR website I would say put down your current college, but if you're in doubt you could phone the IR or your local tax office.

There must be lots of people in this situation, they'll have come across it before. I'm sure they're not too bothered about which college/uni you're going to as long as you're not making it up, which they could check up on if they really wanted to anyway.

The IR website simply says you must "be a student who is planning to continue being a student until after 5 April next" which you will be, so nothing to worry about. If for some reason you weren't, the IR would presumably want the tax paying.