Hey Chris,
I agree a lot with Jessica. I've seen a lot of different types of Bible study formats too and each of them have their own advantages.
The greatest thing I've found in making Bible studies effective is giving the group a feeling that they should be a part of it. Either you need to get them interested in giving answers or you need to give them homework the week before so that they have answers they can freely give. I've been in many studies where some of the people just didn't like talking or were too tired and so they just didn't say anything. Interaction has to be highly encouraged. Keep them all interested and active while still keeping some depth to it. It can be a hard mix.
I like having big groups (up to like 15) when they all stay active and answering, like on a mission trip for example. Usually the devotion studies on a mission trip are much more interactive with a larger group of boys and girls mixed. Those studies are usually great. In most cases though, I prefer smaller groups (10 and under) just because it will keep everyone more active. I would probably actually prefer mixed boys and girls in a Bible study, but I also think the boys and girls do need to have some times on their own, whether it's a weekend deal or a group mentorship time where the genders split. Being able to talk as guys and girls is very important, even if it's not every week. Individual mentorship time is a different matter.
Maturity is also something to keep in mind. In Delaware, we didn't allow kids into SSA until they were 12. That age limit helped a lot because you can get more mature answers and content in a Bible study/discussion that way. We also had a mini mime group that had their own Bible studies, but those were much more structured and much less deep, though they loved them. In Branson, they have a hard time sometimes because kids can join here at 10 years old. A ten-year-old could have a hard time keeping up with the depth of thought of an older teen.
I hope that made sense.
