There's also the distraction factor. Single-sex groups can meet every week per month except one. Once a month, bring together two groups to make a large co-ed group. Also, plan a celebration once a quarter. Ingredients of a Great Small Group Small groups for youth generally work best with three to fifteen teens.
Smaller groups are good for high-accountability while larger groups are great for fellowship.
Beyond fifteen students, communication is hampered and usually only the most talkative will share. Keep the seating close and in a circle so every person can see everyone else's face. The leader may also need to arrange transportation for those who don't drive. The leader is usually more than just a facilitator. He or she is a shepherd, checking with members between meetings, caring for them, modeling Christlikeness, discipling them, and training those who may someday lead other groups.