Match each person in the class up with someone else, a "partner". They can either be side-by-side, across from each other, or on opposite sides of the room, as long as they can see each other (use of a mirror may be required). One person becomes "Person A" and the other "Person B".
Brief them on what's about to happen before proceeding with the drill.
Each person will increase their tension to around 75% while maintaining pace and keeping an eye on their partner. "Person A" will have the lead. When they feel tired and want to take a bit of a break, they'll stand. At the same time, "Person B" will follow the lead and stand as well. Same with sitting back down.
Run this drill for about three minutes: more/less depending on the amount of tension you have them use. I also add that each leader must stand at least three times - the length of the standing portion is their own choice.
After the set amount of time and a recovery period, they can switch leads and repeat.
This is a nifty little time waster, a good way to get people to socialize, and it's also a good tool to motivate people to try harder and challenge each other.