It’s important for kids to learn to do as they’re told, and that their worth is determined by how well they learn the things that those in charge deem important. And your worth as a parent is determined by how well you instill this into your kids. Corporal punishment is effective I find.

Seriously though, school grades are not that important. Are they enjoying learning? Are they learning and doing things that interest them? The only time they should worry about grades is if they need them as a prerequisite for uni or tech courses they decide they want to do. Them, not you. By that age they should be independent enough to work that out for themselves.