I'm not sure that you are correct about people not valuing maths and therefore happily sending them to music lessons instead, Proudmama.
In the case of highly able kids, its more that their learning progression looks like this:-
learn it quickly - half work it out before the teacher has finished her first explanation
practice it. Quickly get the hang of it.
Finish the questions on the worksheet. Ask for another / extension work.
Finish the extension work quickly and accurately.
Next day. Do it all again.
Following day. Listen to an alternative explanation. More repetition.
Two weeks later, finally move onto something new, only to quickly get that and then spend a week or two practising your new skill until your only target is to see if you can do it even quicker than your previous best.
DS2 had 4 years of this. he tried everything to meet the teacher's requirement that he demonstrate that he knew something before moving on. Sometimes there would be competitions in which he'd not only beat the other kids but the teacher too. Everyone (teachers, other parents, TAs, HT, other children) acknowledged his ability but there was just no way the teachers could differentiate enough to keep him even slightly challenged for even half an hour every week.
I didn't arrange for him to go out of class for music lessons etc, but to be honest, if I could have thought of something that would have rescued him from the boredom and the feeling that his needs were unimportant, then I would have done it.