In drywall or plaster walls, position jacks 12 to 18 inches above the floor. If the schoolrooms have wide wooden baseboards, talk with your facilities administrator about installing jacks there. Plaster tends to crumble when you cut into it.
Wood is more solid.
Also, mounting screws don't always bite securely into the wood lath behind plaster. Don't place jacks where they will interfere with the trim around a door or window.