The first thing is to find the reasons why they are underachieving. The reasons can be external: Do they have bad family relationships that influence the students? Is there any school bullying that always distracts the students? These reasons are often complicated and take time to solve. Only by helping them solve their problems can they start to study whole-heartedly. If the reason is internal: like they just don't want to study, or they just can't understand the lessons,* I think this situation is not a big deal. They just need more time to figure out their interests. *This case, you need to merge the sentence because of the "if" at the start of the sentence.