I think the first thing to help them is to figure out the reasons why they underachieved.
If the students wanted to get a good score and they studied hard to achieve it but they failed, I think tutoring classes can help them to better understand lessons. We can also let them to communicate more with the top students to get some effective learning methods.
But if the students they underachieved is because they didn't like even care about study, I think the key point is to lead them to find the motivation for study first.