Green18:
I think you need to talk to the class teacher. First, all you know is that your DC is not completing the work - but you don't know what the teacher is seeing (too much chatting in class/ day dreaming/ struggling).
If it's the first two - well I'm afraid the solution is simple - concentrate more in class and there's likely to be no problem.
if it's the latter - than perhaps talking to the teacher might help you see this a bit differently....
I don't know....of course....but just being Devil's advocate here....
Your DC was getting generous evaluations of Maths performance previously or perhaps up until now Maths work was very easy for them and the new teacher is finding that for whatever reason right now there are real problems with calculation skills/ recall of multiplication facts/ the next step/ etc....
They're giving up their break as well - and working quietly with your child to help them get over whatever hurdle there is...
Again - I don't know that is the case - but it could be (and I hope it is)...
having been at a school where teachers didn't do this kind of thing - perhaps rather than seeing it as a punishment - see it as someone on your child's side - they believe that your DC deserves the help/ support to learn how to do whatever (let's say long division/ maybe solving simple algebra equations) but they're struggling to make the transition to the next step in maths easily.
having been at a school where it was very definitely left to parents to work out what the problem was and extra help from teachers was unheard of - I wouldn't presume this is 'a punishment'.
again - I think you should go and talk with the teacher. I know that break is important and many kids can't wait to get out and run round with their friends/ play football/ go on the climbing frame/ etc.... But... learning to divide is important too. And maybe sacrificing part of break to help get through whatever struggle there is in maths is also worth it.
HTH