As they do the activity, have the rest of the class do something annoying, like talk to each other during the song, or throw papers at the person every time they write their name. Other activities are possible. Then have the student sit down and join the class, have another come in until they have all gone. Ask the students who went out of the room how they felt. Explain that this is a real reaction to a situation.
Improvisation can help an actor practice reacting in a real way.
They have little time to do anything else. Explain that improvisation can help actors think on their feet. Turn the lights out and have the student spotlighted. Explain that improvisation can help actors feel more comfortable in front of an audience because they get used to creating for groups. Extra benefits receive bonus points.