In any normal school there is a large range of abilities in each class. Either the Rebbe can aim at the top, middle or bottom thirds of the class, leaving the others bored or lost. Or the parents can take the slack by either learning with their son themselves to keep them up to class standard, or if they can’t/won’t, by hiring a rebbe.
You can’t expect miracles of your class Rebbe. There is no way he can give a shiur for all levels. Unless more than a third to half of the boys need extra tuition, I would say this is perfectly reasonable.
Of course in a kehilla which has several schools, if your child would have needed extra help, there is always a possibility to send to a less prestigious school where the child will manage with the class. So basically you’ve got to sacrifice either time (learn with your son), or money (hire tutor), or kovod (send to non-top school).