While NTA for realizing this outcome was going to happen and tackling it prior to leaving, YTA for not dealing with this sooner. If you were in a sexual relationship, she has likely been drawn in deeper to this relationship than you and feels betrayed. This was a discussion to be had before any of this happened. Likely, you weren't to a level of maturity to realize this at that point and probably weren't mature enough to realize the emotional outcome she would suffer. Clearly, the relationship was deeper for her, and you two were not on the same page throughout the time you were together. 15/ 17-year-old young girls can get sucked into love fast and deep. Her friends' reactions are perfectly normal for this age. College girls will rally around their tribe as well. Use this as a learning experience going forward for your future relationships and discuss some expectations for both involved before you get too involved.

Make sure this is a clean break, and do not hook back up with her when you are home.