too nice vs being used??

I need hella advice. I’m a junior in high school and I can’t seem to find the line between being nice and being too nice to the point where that person is dependent on me to always help them out and will get mad if I don’t. It seems like people are only making friends with me because I can send them my extra notes, or will do most of the group work, or will always answer their facetime for algebra help. This is especially happening with the boy I currently like- he’s not exactly a good influence and he’s copied my answers several times. How do I cut this off and make more genuine friends??