First, I am working from the perspective that the following are true for your ministry... and two myths need to be broken. I really hope in some strange way, this answers the questions. Hold on! And I apologize for the length of this entry up front, but please keep reading.
First the perspective!
the breakdown: All your leaders have gone through some type of criminal background check/child abuse check. If this is not true, do you know the times we are living in and what is holding you back from keeping your leaders and students safe? Also, you are committed to reaching students through small group discussion with middle school and high school groups meeting separately. The "large group of all the students together all the time for every thing" mentality did not work when I was in youth group in 1980. Why would we believe it works today! This isn't how we educate our kids in school and it's not effective for our students in a ministry setting.
Second, the myths.
the breakup:
Myth #1: "Small groups shouldn't be too small. If we don't have 10 kids for a group we should keep all the kids together." In my experience this myth is created by adults who are scared to meet with students. Some of my best youth group moments have been with mentoring two or three guys in a group. If you have two kids to a group with one leader. This is a great way to begin small group discussion. If you want the small group discussion to be more meaningful, break your kids into same-sex groups by grade (middle school students in their groups; high school students in their groups). In our ministry, we have 6th grade girls in one group, 7/8 grade girls in another group, middle school guys in a group, etc.
Myth #2: "Because we have such a small youth group, we can get by with one or two leaders who rotate in a schedule." I think you need one leader for every 5 students who you have, and if you have wide age ranges to cover, find committed leaders for each age group. I think sometime we get non-committed leaders because we don't require enough from them; we don't want to trouble them and so we set our vision and expectations too low. I thank my leaders all the time for their time in changing kids lives and making a "kingdom" impact on them. If you "pitch" your need with positive conviction and a powerful vision, people will want to join you in your efforts to change lives. I actually believe that you may need more volunteers than what you currently have no matter how big of small your ministry is...
Bonus myth: "I don't have any student leaders who can lead the lesson. They can lead the game time but I really don't have any "spiritual giants" in my ministry." Generally, I think we are afraid to let students lead because we don't want them to spew any incorrect theology or information to other students. Or, they may get kids too excited and out of control which will bother the "Willing Workers" class that meets next door. If you have a wide range of ages, what about taking a risk and letting a senior lead a few freshman in a discussion... with an adult consultant somewhere close by if not right there. Or, what about an 8th grader leading a middle school group? What about letting two 6th graders leading a 5th grade class? When did we lose confidence in our students? When did we stop being the risk takers that all of Christendom believes every youth worker is any way? I say, let the students lead and you mentor and coach them in a small group meeting time before the Sunday lesson time.
Too long to share an experience I had this summer at another church out of the state... suffice it to say that they made a bold decision to invest in a few student leaders and their youth ministry exploded in growth and spiritual awakening. And, as scary as it sounds, not many adults were hanging around. 400 students and 3 youth pastors; the rest were student leaders.
the breakdown? learn to "break up" your group into the body of Christ and watch what the Holy Spirit does in your midst. Take some risks this year and find peace!