Well, it all depends on how professionally you are wanting to record.
If you have or are planning to have a lot of hardware, microphones, and instruments, you will need more power. but if you just new one mic and maybe a guitar input, you can use just about any newer model Mac, even a MacBook Air.
If you decide you need more power, the entry level iMac would suit you just fine. If you need to go mobile, I'd suggest, at least, the entry level 15in MacBook Pro.
It seems like you aren't very serious about recording, you just want to dabble in it, so realistically, any Mac will do. But plan for the future, if you think this is something you might put a lot of time and work into, spend the extra dime on something worth while.
Screen size is also an issue. I have edited music on a 13in MacBook Pro, and while manageable, definitely not a long term solution and, personally, is only considered if I HAVE to edit on the road.
Serious recording = any new iMac or mid-level MacBook Pro
Hobby recording = just about any late model Mac
Well, if you want it for GarageBand AND school, then you're probably going to want a laptop as portability is essential. Any new mac should run garageband just fine, and I'm assuming that since you will be using GarageBand and not investing in a more professional software, you're probably not doing anything too hardcore. Any laptop that fits your price range should do the trick, but you may run into some limitations with a MacBook Air down the road.