You didn't say whether or not there is a family computer at home, but you said you were on a budget, so assuming the new Mac is going to be used by you pretty much exclusively, you're going to need a desk or table for the iMac.
If you have a good place to set it up, I'd go with the iMac. This way, your family might be able to take advantage of your generosity, and use it once in awhile. You'd get more power, and there would certainly be less wear and tear then there would be on the laptop.
Being a teacher, I don't know of a high school district that allows students to bring their own laptops to use in class, due to the fact that while the teacher is lecturing, demonstrating, leading a discussion, etc., the student could be on the internet unsupervised. This could lead to legal problems for the school district.
You can always just bring a flash drive, and transfer your work back and forth to an approved computer at school.
If you're going to be working in your living room, kitchen, etc. and need the portability around the house, save the $200 and get the MacBook. But I wouldn't worry too much about needing to bring it with you all the time.
When you get to college, THAT's when portability becomes an issue. For now, not so much. Whichever you choose, enjoy your new Mac.