If you have to guess if they are underage or not, they are underage. It's people who do not follow this who are people with good intentions but labeled a pedophile by the law.
Honestly OP, just give it a year or so. A lot of high school girls, think it is amazing dating some in college - irregardless if the guy is 28 and has been going to a community college. I would give her to at least 18 and some change.
My limit? 21 or over at least. Has been working out fine for me, I think my friend's girlfriends would drive me crazy. 3 class a semester, photography majors and complaining how "awful" society is.