I think that underperformance is a result of underutilization.
Because when students aren't challenged enough (underutilized) they become bored. When study results are sufficient without having to learn how to learn, this will give problems when students have to do actually something for their school career later on. The motivation for learning drops, because learning to learn that isn't activated in the students brain yet, but teachers are expecting students to administer that already.
Students think they're not good enough to internalize the study material, which leads (after the demotivation to do anything for their school career) to underperformance.