Can you just sing and not dance?

Of course. If you are performing with your classmates, it would be better to have interaction.

If there are three people, you can simply pose a few shapes. It is the kind of combination of motion and static (for example, when A sings, B and C are in the same posture), and then when three people are together, they can move back and forth together, or beat the time in place, or even shake hands twice to form a circle. There is also the kind that signals each other, that is to say, watching each other high-five or something. Is how to be natural. Come as comfortably as possible.