Is she trying to get me jealous? Does she have a thing for my boyfriend?

I have a boyfriend and it's been 3 months since we started dating. He confessed to me and I kinda just went with it, but now I really love him. BTW-I'm in high school. Anyways, there's this upperclmen and she would sometimes come over to our cl and flirt with my boyfriend and ruffle his hair, hug him, and stick to him like glue. This went on even before we started dating. I know that my bf doesn't like her since he's really serious with our relationship and I heard some rumors that she already has a loving boyfriend. I was a little irked at first, but always tried to leave the room whenever that happened (maybe 2-3 times in the 3 months) because I just couldn't stand the atmosphere. I'm not jealous, but a little annoyed. Another thing, I'm seriously bad at dealing with these things...At first, we kept the dating thing a secret, but as time went on, people started to notice us hanging together, so....they know. But she doesn't know, maybe, she only knows that whenever he's there, I'm also there. But honestly, is she that dense to not read the situation? Or is she doing it on purpose. That was just how I felt until yesterday. After school, some of us stayed behind to catch up on work. Then she comes in and flirts with him. Well, I thought this was just the normal routine, so I didn't really care much. But, next, she forcibly sits on his lap! But, because of my old habit of hiding my emotions well, I just looked away and few minutes later I made an excuse to leave the room. Then, my friends who have a hunch that we're dating says, "Are you okay?" or "Who is she?" or "That was just going to much." Anyways, after that, I gave my bf the silent treatment. Really, couldn't he have just refused her?! That's what's bothering me. So, what should I do???? Yes, yes....I know, I should tell my boyfriend to stop her, I'll do that on Monday...But, how should I deal with it if this happens again? BTW-I'm seriously not the kind of person who can just throw out all their rampaging anger at someone...I'm quite shy so....