Energizer

This one I recommend. It always works, if you believe in it yourself. It can be done only if the number of students is even. If not I suggest you give someone the runaround. Certainly the headmaster needs urgent information, whatever.
Act like the severe strict teacher the kids know you can be. Pass on sheets of paper, one per pair of students. Tell them to clutch hands. Do glare fiercely at the two boys who don't like sharing hands until they follow suit.
Explain that each couple now has one pair of hands available, a left one and a right one, right? Tell them you are going to show how to fold a paper air plane. They are to co-operate with their pair of hands to copy your procedure.
Turn a blind eye to students who swap now: left-handers have to be expedient in a right handed world.
Be sure to be able to show how to fold a paper plane model that is somewhat beyond their ken, that they haven't seen before. I f you have never folded any paper plane, then leave this post. Try something more serious.
Of course, as the teacher you are, you understand that you have to demonstrate this above your head against the blackboard or whiteboard with your back towards the class, to prevent any confusion about what's done left and right. Don't crick your neck when looking over your shoulder to synchronize the moves. End with counting down for take-off.
I believe I invented this activity myself. But I'm not too sure about that. As an artist I am an expert in nicking ideas unconsciously.
This year I started with a cover lesson. A colleague of the maths department had taken ill. There is a lot of mathematics in folding paper planes. I bet you can come up with that yourself.