I don't want to help a friend with any of his school work.
Why don't you want to help your friend ?
He takes up too much of my time.
Part of being a friend is sacrifice. However, it's understandable that you may need to reduce the amount of time that you have to help a friend.
School is too much.
If you find that your friend is taking too much of your free time, it's ok to reduce the amount of time that you use to help. While it's good to help friends, you aren't expected to neglect yourself.