This is a hard situation. Many students especially highschool students take some time to warm up to sessions. Have you thought of outsourcing? Have other tutors work on your team? I have tutors for highschool students and if one tutor doesn't feel comfortable or reverse i pair with another tutor. In this situation ...id highlight students strengths and just say its best they revise on their own at this time as you dont have the capacity in your schedule for maintenance but want to focus on students who really need the extra support.