Activity - LOTS of activity! Teens have a lot of built up energy and need outlets. Lots of board or active games, time for games and "acting" type mixers.
Of course, there should be a Bible study at a fixed time and place each week. Free time before Bible study should be unstructured, but MONITORED! Teens WILL "get into trouble" if there are no boundaries, but they do need a certain amount of free time to just blow off steam. Game tables, ping pong, air hockey, whatever....these are very popular. Anyone interested in helping the youth of your community may be able to donate some of these items.
Next, you must schedule activities away from church - together. In other words, plan at least ONE A MONTH activity....whether it's movie night, a trip to a pizza joint together, mini-golf, bowling. If you have teens with driver's licenses (we don't!), you could schedule a "Progressive Dinner," where you travel from one house to another....at the first house, you have hors d'ourves (spelling is wrong, I know!). Next house, maybe soup and salad, next main dish, final house - dessert.....just a fun activity where the kids can move around a lot.
Schedule a treasure hunt (I'm sure some ideas can be found on line).
What I'm saying is, teens need to be kept busy! In addition to teaching scripture, give them lots of alternative activities to spend time together in an unstructured atmosphere with the "leaders" so they can be free to approach you about anything that is on their minds. This also gives you lots of unstructured time to evangelize any kid you feel is not saved, or needs someone to mentor them. Build relationships. That's what Jesus did with His disciples!
Before one of our dear friends moved away, he built up an impressive youth group (close to 100 kids attending every week in a town of only ONE H.S. with 1000 students!). A very small town, lots of other churches running youth groups for many years, and NONE of them built up a youth group of this size! I believe his success in doing this is due to the amount of "out of church" activities he scheduled every month. AT LEAST ONE NON-CHURCH ACTIVITY was scheduled each month. This showed these kids that the pastor cared as much about their physical and emotional health as he did for their souls. He visited the school, and sat at the cafeteria during lunch once a week. Kids got to know him, and then they wanted to come visit our youth group.
Let us know how your venture is going, and God Bless You!