A lot of teams meet once a week during the fall to prepare for the build season. We have MOE University, where the students work with mentors in their chosen subteam to learn the basics. If you don't want to divide students that early, you could have one module a week on a specific field - electirical, mechanical, programming, etc. - so that the students get a feel for each and can decide later which area they prefer.
We have also done special projects in the fall, in order to teach the students basic skills and also to build teamwork. One year we did balloon launchers, another year we did Rube Goldberg devices. There were 4 teams who competed at the end with their projects.
If you don't have enough mentors for a MOE University type program, perhaps the more experienced students could each research and lead a module.
Many teams have similar programs; here is link to ours. Let us know if you need more help!
http://moe365.org/moeu.php