The most common type of plastic golf balls are plastic dimpled and plastic with holes and HDPE foam types are legal. As long as they are commercially marketed as "plastic practice golf balls", they may be used. It is recommended that students bring the packaging if there is a question.
Rule 4.d. explicitly says that the target is either tape on the floor or panels lying on the floor. There should not be any significant difference between the floor height in the launch area and the targets.