I'm gonna breakup with my boyfriend so I can date one of his friends instead.
Do you have a good reason for breaking up with your boyfriend?
I've been thinking of it for a while now because I constantly see him sitting with a friend of mine at school.
If you want to end a relationship, you are completely in your rights to do that. However, it's generally not a good idea to date someone who is friends with your ex because it will be awkward for everyone involved. Try to date someone not connected to them instead.
But I want to see him cry when he sees me with his friend.
It's common to be angry at an ex after breaking up with them, but doing something to make them cry is needlessly cruel. It will also give you a bad reputation among your friends. Try to consider your ex's feelings.