Ignore when possible
When it’s just a couple of teens talking a bit too loud with each other and the rest is still listening, ignore it. Usually, they’ll stop after a bit.
An effective and subtle way to address the disruption is by using silence. Just be quiet for ten seconds or so. You don’t even have to look at the tens making the racket, as a matter if fact I would advice you not to as to make it subtle. Their noise can be heard better and nine out of ten times they’ll notice and stop, or their peers will ask them to.
Make a joke
A good way to get disruptive teens ‘back’ is by making a joke. Mind you, not a joke about them, but a joke in general. The laughter will interrupt whatever they were doing and will make them pay attention to what you’re doing.
Get their attention
Getting the audience’s attention in general will often stop disruptive behavior, like doing something, changing something, telling a story, etc. I’ve written a post about how to get your audience’s attention (back) while preaching, you could use any of these methods. It will focus the attention back on you and your talk, hopefully ending the disruptions.
Make eye contact with a leader
Most of the times there will be leaders in the room as well. If the disruptive behavior is bothering you to the extent you have trouble staying on track, see if you can subtly make eye contact with a leader. He or she can then walk over to the teens to have a word with them.
Don’t admonish publicly
Normally, I wouldn’t advise you to admonish the teens that disturb your talk publicly, for instance during your talk. That’s quite a severe approach that can have lasting effects. The teens may feel so humiliated that they’ll never come back.
If the disruption was severe, or if you see a pattern emerging, talk privately to the teens involved or have another leader do this, for instance their small group leader. Usually i-messages work well here because they’re not accusatory and explain what their behavior is doing to you. If it’s a pattern, it’s important here to get to the ‘why’ of the disruptive behavior so sit the teens down for a good talk about what’s wrong.