Help please

So im in year 12 at school and iv been dating my boyfriend for about 7months and even before we dated there is this girl and she was obsessed with him and even though we are dating she continues to obsess and my girlfriend who hangs out with the girl that obsesses had a birthday party last night and invited my boyfriend litterally for her and he went! i was mad but i didnt try to stop him and he told me she was trying to hug him and sit on him and i want to punch her in the head but i dont know what to do 😭