I think the teacher sometimes says and explains things to the whole class, and if what the teacher says, or homework s/he has prepared, applies only to 5 out of

I think the teacher sometimes says and explains things to the whole class, and if what the teacher says, or homework s/he has prepared, applies only to 5 out of 25 kids, the others can get bored and unmotivated, sometimes even start disturbing the others. And sometimes the kids talk with each other about the maths problems. But that won't work so well if they're in different books. Simpler for both the teacher and students, if those in a class are at roughly the same level. But I do think that what you say, would work in a group where everyone is pretty good at maths, enjoy it so much so "yes I got a new book" they think (when they get the next book). And dive into it, no matter if there's a teacher there or not. That's not generally the case though is it