I think it is harmless as long as she knows what is happening (that you are not really pursuing her)and you don't mind showing her how a man is suposed to treat a woman properly on a date every once in awhile...you could raise her standards and teach her what it means to be treasured and respected. I dated a 24 year old when I was 18, but he was very careful and respectful of me. Yes,he got some crap from some of his friends, but he didn't mind because he had permission from my father and I knew and he knew that what he was doing wasn't harmful.
Honestly,when it comes down to it, you need to know your identity, who you are as a person. If you know that you are not taking advantage of her and are going about things in a respectful caring manner, there is no need to worry about what other people think of you. There is no need to fear the opinion of man. When does she turn 18? It sounds like the ex is causing quite a rucus at school and most likely lots of stress at home...Whatever you do, don't do it if you don't have peace first.
Blessings on your decision.