All glider landings will take place on the grass runway. Unless specifically approved by the Chief Instructor or in an emergency, no landings will take place on the tar-mac runway. Generally, one or two gliders on the grass does not constitute an emergency. There is a lot of area and you should be able to land with little trouble. The grass is not exclusively a glider landing area.
Power planes can and do use the grass for take off and landing.
In the case of a power plane staged to take off on the grass. It may be wise to land on the tar-mac if you are sure it is clear to do so. It is important that you clear the center of the runway so that others may land after you. Once stopped, get out and move your glider to the side of the runway. Do not wait for a crew to assist you, most pilots can move even the largest glider the distance required to clear the runway.