Hey Teacher, don't leave the kids alone. Make your lecture attractive to attend to, instead.

Pictures from the Demonstration against the cuts over the Student Benefits, Copenhagen, 2013.

Here's something you can do.

   Use buzzwords while you've been talking for a while, so that the audience can have something - like an ancer - to drag itself out of random thoughts or what's going on out of the window?! (or be more creative, random thoughts are good, far associations are even better).
Use them even more when the topic is much harder to understand or the audience not so accustomed with.
   That can work counter wise off course, so pay much attention to words you shouldn't use for example 'break' 5 or 10 minutes before break, you've totally lost them. They will be thinking only about break than what you're trying to describe. Maybe you think it's just a word but believe me after a long almost one hour talk break sounds like 'BREAK'
   Be careful of the time that everything evolves. Don't forget your audience is a living organism, it breathes with you,(well not precisely, that doesn't mean they share the same ammount of interest in the topic as you) but they can react to whatever you say, how you move, the pitch of your voice (yes, it is a performance), the time when you say it, so you have to be in constand alert and cooperate with them and observe how this relationship evolves over your presentation time. So, try to keep eye contact with everyone if possible. Be aware of your body language.
   About the presentation itself, use images, videos, interactive tools as much as you can. If it's something practical demontrate it as one. Don't just talk about the theory but Do it. Get people involved and participate. Don't think about the time. People learn faster when they actively do things and get their mind to work on how to deal with tasks. Ask questions. If their awkard ask them to cooperate in a small activity. A quiz for example. Ask their opinion on things or how they like to learn. Encourage them to contribute in the learning tasks.
Well, this more or less and I promise you you'll charm them.

Sincearly,

a very easily distracted student.