I don't think that you should be thinking that your girlfriend is cheating unless you have a reason to think someone is cheating. Let's be for real for reals for reals 110%. Her staying after class is not what worried you. There were other signs. This is the only sign that you want to bring to the table. So you need to come back and be for real about what's really going on with your relationship. There were warning signs and you ignored them. Either she's unsatisfied for you or you're unsatisfied by her and she did what she had to do. You do what you have to do.

call me crazy but is that not the goal or the intended outcome? so she decided to go to her teacher with her bad marks and they stayed up, studied together and you're surprised the grades got better at the end? I would call that cause and effect. I wouldn't necessarily even call that nefarious in any way shape or form

maybe she's just getting some tutoring like she's staying late with the teacher because she wants to get good grades and now she's getting good grades because she's getting extra tutoring maybe from the teacher or help I love how people always think it has to be something sexual this probably isn't even real it's probably a made-up story

Is she really your girlfriend, or is she the teacher's girlfriend? But one day, when they're doing whatever, staying late with each other, bust into the classroom while you're recording, and then post it on social media, and catch her in 4K.