I am the step mom of an 18 year old young man?

We are preparing for graduation in a few weeks. He lives with my husband and I and his mom is not a major role in his life. They had joint custody up until he was about 9 years old then my husband went to court and received full custody. She was on her 4th husband siince they divorced and became very unreliable or dependable. She thought of only what she wanted and how she wanted to have fun and stepson got pushed to the back burner. Many night being left at school for hours as she was to pick him up, getting dropped off at strange mens homes while she went out and partied. There were many, many times he would cry to his dad why his mom couldn't love him, what he had to do to be perfect enough for that. It was at that time when I decided that I would no longer wait for her to do the right thing, I would step in and fill that void if I could. My two children were on the verge of moving on with their lives at this point. His mom has always lived fairly near to us and all she would have had to do was make a true effort to be a part of his life and he would have forgiven her. So I have been the go to "MOM" for a long time and now, with the graduation approaching, she has "seen the light" and wants to step in and be the mom again. She has made a couple friends that work at the high school and has pleaded her poor story on how my ex has brainwashed the poor boy and he won't have anything to do with her. She has made a good sob story I am sure, she is VERY good at that. So now, the school will not let me take care of anyting that needs to be done because I am not the biological mom. My husband is in the trucking business so he is gone for periods of time. He is not always available to do what needs to be done, sign paperwokr, make phone calls, so I have always done it. Well , we have been infomed that this is no longer acceptable. My stepson has told the school that he does not want his mom invovled in his life to that extent and they are basically saying too bad. I don't think it's any of their business. They do not know the pain and misery that this woman has caused and I neither have the patience nor the time to sit down and try to explain everything and I have ALWAYS done this. This is a woman that missed Mothers Day Programs year after year, if I hadn't gone he would have had NO ONE, hasn't been to 1 Teachers Conference since 3rd grade, no plays or school activities, unless it made her look good. I guess I am just frustrated and want to know what I should do? Stuff needs to get done and it won't if we wait much longer.