Our team's student handbook:
http://www.redalert1741.org/go/handbook/
Before build season, we have minimal requirements of participation in our outreach events. We also offer training opportunities, and attendance at these really helps sort out who are the committed team members.
Realizing that some students are able/willing to commit more time than others due to various reasons, we have a tiered system of requirements. A general team member is required to meet 60% of our scheduled time during build season; sub-team captains 75%; main captains 90%. We officially meet for 24.5 hours per week, though in reality it's somewhere between 30 and 40.
During competition season, we meet less time per week, only scheduling 14 hours, and the percentage requirements are reduced as well.
There's also an academic requirement which is stricter than the school's policy on extracurricular participation. We stress that academic success is paramount and if they aren't putting in the effort in the classroom, why would I expect to see the requisite effort in the shop?
Lastly, we offer varsity letter opportunities. This is more of the carrot approach rather than stick. The criteria put emphasis on participating in all the integral parts of our team: outreach, build season, training, and competitions.