If you're worried about the boy taking advantage or even just pressuring her to do anything she doesn't want to, make it very clear that you're open to any and all inquiries she may have.Also: under any circumstances whatsoever, when it comes to her body "no" means "no", and she is in control of what she does and doesn't do - not someone else's desires/requests.
The more you try and control the more they'll push the boundaries. That’s when they start learning about their future and what not to do. Okay as a senior in highschool, we would consider the guy in the equation to basically be a pedophile. Senior boys aren't so good for naive young freshman girls. Your daughter will be upset but trust me, you'll be thanked for it later. I have had experience with a guy from 9th, 10th, 11th, and now 12th and it seems to be that the juniors are normally the ones worse for freshman girls.
My mom had issues with it at first with the whole age thing, but once she got to know him she was okay with it.
It took a little while, but now she views him as her second son and we plan on getting married after we both graduate college.
Just talk to your daughter make sure she knows what it is that is concerning you so much but don't try to push him out of her life, more than likely she already knows she may only have her freshman year with him.
Their will be peer pressure on both sides so she doesn't need the stress of her parents not liking him to add on to it.