unsympathetic school teacher - should we get involved?

[bathmummy]

My DH has a son from a previous marriage. He is now almost 7, a little wild but essentially a lovely little boy. He doesn?t visit too often, DH goes to visit him with our children (his mum doesn?t want me to be involved at all unfortunately).
However, she is having real problems with his school and the school teacher over a school trip. Apparently he is too naughty and has got too many "black stars" on the class chart and is not allowed to go. Apparently there is no way he can redeem himself by getting yellow stars so there is no incentive to behave anymore and so he has "given up". He doesn?t want to go to school anymore, has been loads of incidents but seems that this trip is either the straw that broke the camel?s back or even the main catalyst for new bout of behaviour. He has an IEP but pretty ineffectual - he is bright and knows what to say to keep along the line when it counts.
DH is livid and wants to phone the teacher, but his ex wants him to stay out of it and "leave it up to her" - which is fine only the situation is just worsening and worsening. DH doesn?t know whether to get involved against her wishes and try to help sort it out or to leave it and see him go downhill. I guess he probably will phone today but just wondering how others manage joint custody with the other parent with main input when it comes to schools - will he even be taken seriously and how reliable are they are not informing the other parent about the discussion? Should he write/phone/visit? Ideas please
sorry for long rambly post, not slept well as DH tossed an turned all night thanks to the worry.

Why doesn't the mum want dh's involvement? At the end of the day, he is the child's parent too and should have as much involvement in his upbringing, where practicable. Surely the dad's involvement would only be a good thing. I can't see how it would make things worse.

If I were your dh, I'd contact the school to see if the matter can be resolved. A fresh pair of eyes on the situation might see a solution not apparent to those already involved.

From a teacher's viewpoint, the school is probably right not to take him as they are responsible for him. I would not want to be responsible for a child whom I thought might cause trouble outside of school. Unions advise against any trips so would worry about support from them if something did happen.

With regard to his input- afaik he can request to be kept informed although it might be difficult from a purely practical viewpoint.