The area around the classroom and ceiling is:17 * 6+2 *17 * 4+2 * 6 * 4 =102+136+48 = 286? square meter

The powder coating area of two walls is 6*4*4=96? square meter

Door and window area: 3* 15=45 square meters.

The area occupied by the rainwater ceiling around the classroom is: 0.25*4*4+0.25*6*2=7 square meters.

So the coating area is 286+96-45-7=330? square meter

So paint 330 * 0.2 = 66kg is needed.