You have a "Misc Volunteers" team that is set up to accept all ages. (If you don't specify ages, the system interprets that team as being open to all.)

You really shouldn't need a team that will not have students. Volunteers who are not team leaders do not need to be on a team. 

Though I don't really recommend it, if you do want to keep this team, you can set the size limit to -1. That should prevent anyone from being automatically assigned to it.