Eh, this is pretty much a cultural thing, not a linguistic thing. Where I'm from (in Michigan), referring to adolescents as "kids" is perfectly normal, while referring to them as "children" can be kind of demeaning. But at the same time, "child" is often used to mean "minor" (as in, under the age of majority, which is 18 years). So it's really context sensitive.
If you're not sure, "student" is always perfectly fine. You never need to use "schoolchild" or "schoolchildren".