I have to admit I don't know if it's compulsory to attend.
However, my DD is in Year 9 and did her third maths challenge yesterday. She's in the top set but not a genius. I know it's not as important as GCSE results, but personally I don't mind her being entered, she's already achieved a bronze and silver and if she gets the chance of more awards it could boost her confidence and it's also something else to say she's achieved at school.
After the papers were taken away, she said that they talked through most of the answers, which I think is good as they know where they've gone wrong straight away and can learn from it. If your DS's shool does this, it could be brilliant for pointers on where the concentrate on his maths.
She's given me the impression, it's something quite a lot like the thought of doing and they support eachother. They know which two in their class are most likely to get a gold and her and others are looking forward to seeing if they achieve it. My DD is also hoping that some who are very bright and for whatever reason haven't got an award yet, will this time.
The school and teachers of other subjects will know this and other things crop up over the year and they have to release pupils, so they shouldn't have an issue with it - it might be their subject they want children to attend next time.