One of the first and most difficult things for traditionally trained novice improvisers to do is to leave behind the fear of making mistakes, of not being perfect. One thing that may help this process is to change venues. Instead of playing in the same place (i.e. classroom, recital hall, etc.) where perfection is the goal every other day and hour, take the improv games to unusual (or at least different) environments. There’s something about the association with the old environment that hinders the “letting go” and something about an unusual environment that helps one play a new way.
Instead of playing on the stage, go out into the seating area of the hall. Scatter players around it, including the balcony. Or play in the furnace room. Or in the lobby of the concert hall (but not when another concert is going on). In a classroom. In the gym. Or outside (weather permitting): in the parking lot, under a tree, on the sidewalk. Or move the whole enterprise to a completely new place, such as the city park. The pedestrian zone downtown. An art gallery. Under a bridge. A playground. A basement. Near a big fountain. A plaza. Along a lake or river. In a stairwell. A hospital (big ones may have large lounges or open areas and are glad of the ambient music – but clear your appearance with their schedulers). You may have to seek out locations that offer some small measure of privacy for novice improvisers, but as soon as they get over the need to maintain an external image of perfection, they will be able to play nearly anywhere.
If you can’t change locations, experiment with other changes: wear masks, animal costumes, outrageous facial make-up, blindfolds, paper bags over the head, wear sheets like costume party ghosts, silly hats (then play the hat!); stand on one foot; trade instruments among the players; turn out all the lights; sit on the floor, lie on your back, face the wall (up close); stand (or arrange the chairs) in different formations (e.g. a circle facing out or in, in a checkerboard fashion filling the room, in random positions facing random directions, stand back to back in pairs), or adding a physical movement (swaying side to side, back and forth, bending at the knees, moving the instrument in a circle, waving arms in the rests, etc.) either the same movement or independently chosen.
In any case, do something to alter the usual venue or experience to help open the minds and free the attitudes of the novices. Fear and fun can’t coexist. New place, new mind.