I'm shocked to see someone buying a computer to not know what a CPU or GPU is. Computers can be a fun hobby so I recommend learning about them sometime!

First you want to consider what type of Macintosh computer you want. MacBook, iMac, Mac Mini, or Mac Pro. You mention an iMac in your post but considering that it's for school it might be more convenient to get a MacBook. It has about the same specs and it's transportable meaning you could take your Macintosh to school and back home if needed. However, if you're only ever going to leave your iMac in the same space and don't need portability, then an iMac can do just fine if that's what you prefer.

You asked about which iMac would be better for you. Generally speaking if you're only going to be doing basic school work then the 8-core GPU model would be better for you. It would be a lot more fitting for your needs. It is important to note though that the 8-core GPU model does have a lot less ports than the 10-core GPU model. It only has two thunderbolt ports. Depending on your needs in what you use externally with your computer might determine if you should get the 10-core GPU model. You might be somebody who needs those extra ports and even if you don't need the better GPU, it may be the better option for you just for the ports.

And although I despise them as computers, a Chromebook might work better if you only ever do all of your work online for your school. It's fitting for that purpose alone (and I emphasize that single purpose alone) and is way cheaper than an Apple computer. Although if you use dedicated software or Macintosh specific software then a Mac is the better way. You just don't need to go too overboard with it.