Seems like you've already made up your mind.

I've been in similar situations a couple of times and every single time I went through with it. I had sex with several girls who at the time had boyfriends, unfortunately I was usually friends with them before hand and not anymore after. I have personally split up 4 relationships in less than 3 years. I have no regrets, these relationships were over before I had even started putting my moves on them, I just gave them the final push so what it gets you is

a) you get to have sex

b) you are ruining a relation ship

c) the girl will put the blame on you and only you because that is how they rationalize that because no matter how hard you have to try to get laid by her it is her lack of judgement and it is her saying 'Yes.' to you and her decision to cheat on her SO. It will still be your fault and yours only for life. She will tell her friends, she will tell her family, she will tell everyone you've ever known the same unless she is mature enough to accept the responsibility which in all honesty I doubt.

If you're ready to take the fall for having a quick orgasm, get laid but trust me, it is not worth it, you will still feel like shit and it will come back to bite you in the ass.

On top of that, you are likely to never see her again after that, she will ditch you 100 %, can you live with that? And even if she didn't dump you, would you actually consider dating such a person? What if you're at work for a month and she feels the same again? 'The relationship is falling apart.'

Make your move if you want to but don't expect to be proud about it, in fact, no one should ever know. If your school ever meets for an anniversary you can assume that everyone knows about how you destroyed a relationship. Can you really live like that?

Don't. Just don't.