I guess the whole "you know what to do" is you're her big brother, you burn this dude. I don't mean literally, I mean you go talk to your parents and instead of focusing on punishing her into paste which is super easy for conservative families to do and often the path of least resistance you help bulldog this thing into your family coming together to protect your sister and make sure this senior student faces consequences. Either they all go over to his house old school and say hey your son got our daughter pregnant, what are you going to do about it? Or figure it out. If she was forced, then you need to talk to the school principal, maybe the police need to get involved as well. But you do something.
back in the day before my time, a senior freshman relationship would probably not include sex. because everyone would be watching them like a hawk mostly, lol.
In this age of teens having babies, the seniors dating freshman where everyone knows what's up, that's pretty obvious and pretty frowned on. Like desperation and 'of course a freshman girl will say yes'.
from the picture the op painted bringing in the parents or the school wouldn't be all that conducive to a healthy resolution to what's going on.