You will never know unless they choose to tell you. Got to let them go. They will soon figure out that going to class more often than not is helpful. But there are some cases where yes…the professor/teacher is not a great lecturer and actually makes learning difficult (very heavy language issues etc) and the student can use that time more wisely by seeking out other ways to learn the material. This actually happens so my attitude is learn the material and do your best but be flexible. The other added benefit to attending classes is getting to know the professor and developing that important relationship. This won’t happen unless you show up.