Sorry you are having a hard time Donna. I think that we all go through similar stuff. My ex and OM bought a big house in our old neighborhood and now she wants to bring him to our kids events. I told her that it would make me uncomfortable and that if she truly wanted to have things comfortable that she should put some work into it rather then just pretend it is. She of course ignored it. IMHO, you can't punch someone in the nose and then expect that time and denial will fix it.
SO...I think if you need to avoid him - avoid him. It's his problem to fix, not yours. Your job is to move on with your life and make the most of it that you can. He can only stand in the way of that. You deserve better and you will have better but first YOU must believe it.
You have a teenage boy. He's going to be a handful, he's going to test his boundaries with you and he's gonna screw up once in a while. He has to - to learn. It's your job to set consistant boundaries with him and let his actions have consequences. Maybe have him send a note in to his teachers explaining why he missed school and have them sign it. Think he'll do it again?
There are plenty of good kids who were raised by one parent. Be happy that their dad is involved but you do what you need to and what you feel is best for them. (((((Donna))))