Leave him. If he comes back or finds you, then he loves you. Just wait to see if he misses you. There's a phrase saying 'You only know you love her when you let her go'.
Just ignore it until they stop unless, it is physical then go tell an adult so that they can help you with your certain situation... Hope that helps!
Support her relationship from far away. When she breaks up with him and only then, you can approach. Until then, you are a supporting character, not the main character. She gets to decide when to move on, not you.
Tell him what you feel. Reject. Say no in a nice way.Don't let anyone thinks you like them too because you will just hurt him/her at the end. Dont let him assume you have that spark.