The thing is mixed ability teaching can work in maths with a good teacher, but it can fail horribly with a poor one.
I wouldn't worry too much with one lesson of rounding to 10/100, myself. Child may complain they're bored, but sometimes going over things can help them to really remember it. However if it continues at that level, and nothing is changed, I would ask if I could set my own work.
Dd1 (year 8)is top of the top set at a large school and does sometimes complain of being bored, even through the work is diffentiated even within the set. However, what I have noticed is that if she races on with subjects in the way that keeps her interested the whole time, she retains less when she comes back to the subject.
So if say she's done equivalent fractions, when she returns 6 weeks later to do adding fractions, she has to take time to get back into doing the equivalent fractions before she can start looking at common denominators to add them. If she's spent slightly longer on the equivalent fractions (and may well complain of being bored!) then when she comes to adding them, she is totally competent at getting to the common denominator and she actually moves quicker on with adding them.
So being bored at times isn't always a bad thing.
But yesterday instead of doing her homework she taught 6yo ds to do it. She wanted to prove how easy it was to the teacher. It actually was very good for her as she had to explain how to do it and why rather than relying on her intuition.