The thing with the iMac is, my current desk can't fit a 21.5 inch screen, mainly because it has shelves over it...
Problem solved! If you desperately need a mac right now, get the macbook.
However.... Only get one if all the other kids take laptops to school. Otherwise you'd be the class geek and get beaten up. Im sure you have a family computer so why bother getting a desktop?
Personally, I think you should wait until you go to college before you get one, Im sorry that this sounds patronising but do you really need one for high school?