I am sorry to say this but she is manipulating you with the flunk issue.I know as a mother you don't want you kids to fail, but from what I stand, sometimes for their own sake we have to be "the bad ones" you can have it both ways. Now that you have "some"support from the ex you should make her came home and if she flunks, well, she is the one that will have to deal with the consecuences, and that's the key in my opinion. She needs to understand that her actions have consecuences that ONLY SHE will have to live with and assume, such failing at school or(God forbides, getting pregnant) Seems like she is trying to hurt you for whatever pain she may have inside, but you can allow yourself to feel guilty and let her to do whatever she wants. If she will come home she will have to leave by YOUR RULES, if that becomes imposile to accomplish then she need to have a clear understanding of the consecuences of her actions. Hopefully, your ex will back u up on that and stop being an enabler...As for the boyfriend's parents, you can't really sweat that one much, right now your priority are your kids, they are the ones that will one day thank you for being loving but also for being though (I know I did)
Please be strong ,God will stand by you...