I'm not saying that smarter people are better people, but if 20% of the class can't multiply properly, learning integrals will be a pain for them, and instead o

I'm not saying that smarter people are better people, but if 20% of the class can't multiply properly, learning integrals will be a pain for them, and instead of learning more, the teachers will be occupied by the 20%, while the smarter ones will be bored, and a portion of class time (opportunities to learn more) will be wasted, since they'll be bored by the stuff they already know.