1) Meticulously teach children the right way to do things. Now you see why Montessori schools don't have much trouble in this area, and why their teachers get paid more. Public schools can't be arsed to do this, and then wonder why they have problems.
2) Feed them. Hungry kids have emotional regulation approaching nil. But since life is a paid privilege not a right in America, only people who can pay for food are allowed to have it. Programs purporting to feed the hungry are woefully insufficient, and school food is notorious for poor quality and nutrition.
3) Exercise them. Encourage walking and biking to school, followed by plentiful recess. After they have burned off excess energy, they will naturally sit still and shift toward intellectual interest. Instead, schools are banning children from walking or biking, and shortening or eliminating recess. This does nerve damage, as the body and brain cannot develop properly without vigorous motor stimulation.
There are, of course, many other factors such as poverty, broken homes, and so on but those are harder to affect from the school direction. The above methods are things that can be done in school.