In America, all the people I know with bad grades literally don’t do their work. On-level classes especially aren’t that hard for the vast majority of kids. The problem with grades isn’t the school’s problem, it’s the parents’ problem. Kids these days are undisciplined and they place education very low on their list of priorities. I know plenty of people who could easily make good grades but they are constantly talking to someone or on their phones, not even trying.
TL:DR: Kids are undisciplined and that’s why their grades suck.