Maybe I now understand. Try this for student class:
import greenfoot.*; public class student extends school { public void act() { move(4); if ( Greenfoot.getRandomNumber(101) < 15 || atWorldEdge()) turn( (Greenfoot.getRandomNumber(2) * 2 - 1) * 90); if (Greenfoot.getRandomNumber(2000) < 5) getWorld().addObject(new afval2(), getX(), getY()); } }