FAHA: Tissue-Thin

I'd say start getting the other "replacement" people more involved with the team. If you can get to the point where things are going well without the mentor, he/she may feel like they can spend a bit of their time elsewhere. However, be careful with your approach. You don't want to sound like these other people are here, but you still need the mentor, and you don't want to sound like these new people are taking over right away, too fast. Give it time. Maybe try this over the summer, or the beginning of the school year next year when your team starts up again.