Many schools will not let you paint murals on the walls because the wall is not suitable for paint, or the administration is afraid of damage or graffiti. One solution is to paint the mural on a sheet of masonite and then affix it to the wall with screws. You can buy these by the sheet at any lumber store and it can be cut to any size or shape. The mural can be removed for touch-ups and the integrity of the blank wall behind the mural is not affected.