Stop spending time with him as you previously did. Just make an excuse that you have some work or that you are looking for your best friend. If you see him in school, then act like you are busy with your friends or pretend that you have a call.
Do not begin a conversation with him under any circumstances. If he starts a conversation then just nod and ignore him whenever you can.
Be less intimate
If your boyfriend tries to get intimate, pull away after a few seconds or turn your head. If he tries to talk about his day, then do not ask questions from him. If he gets upset and want you to pamper him, do not give in to his demand as this would break your resolve to end things with him.
Ignore his compliments and do not reply to them. If he makes a joke, avoid laughing and do not encourage him to make more jokes. Be indifferent as much as you can. Always act weird when you are with him.
Flirt with others in his presence
This is a great idea and it is best to flirt with his close friends. This is something which can really tick off a boy friend and he might actually dump you after seeing this.