If they are under average then that means they have a slimmer chance of graduating. And since it is the teachers goal to help their students graduate, they should help them as much as they can; and that might mean forcing students to be tutored. If their grades are really bad then they might not graduate, and would have to do the grade over again, is tutoring really worse than repeating the grade?
Most kids want to learn; but, there are many reasons that may be stopping them; especially, if they get involved with bad kids who do drugs or alcohol, which is becoming too frequent in grade and high schools. Which is why mandatory drug testing should be required in grade and high school quarterly, using hair samples, which can't be beat. Any, students who appear to be drinking, should be given a breathalyzer test. Any students testing positive for alcohol or drugs needs to be sent to therapy.
Once drugs and alcohol are ruled out as the problem, then teachers and parents need to meet and discuss the problem; at some point the child needs to be brought into the discussions.
If through discussions with the parents, it a appears there are problems at home with the parents, it may be necessary for child and family services to evaluate the parents and the safety of the home environment for the children.
Maybe there is a medical problem and doctors and psychologists might need to be brought in.
But, in most cases the child is just afraid to ask for help.
Tutoring should be mandatory along with summer school for children who are failing.
Many students who are failing, or are not reaching their potential, suffer silently. By making students who are struggling seek help, grades will improve. Asking for help some people may feel it makes them dumb or are scared to take the leap of fate. It is hard sometimes for a teacher to give individual help during a class, but by being tutored that student will get the individualized help they need to understand the material.
Forcing a student to be tutored is not going to help them at all. Poor performance in school is most likely due to habit--if not, the student would willingly accept extra help in hopes of improving. Unless a student actually wants help, they are not likely to retain any information or do any better than they did without a tutor. Extra help should always be available to students, however forcing them to receive it is a waste of time.
One cannot force content into someone's head. If students do not want to learn they will not learn, no matter the method.
Actually forcing them to learn through pressure would have more deterrend effects than anything. Instead, students should be motivated by the teacher. A teacher who cares about his students and understands the reasons behind their lack of motivation will do way better than a teacher who forces them to sit down and shove material into their brains.