Question

consider the four corners of the front wall of a rectangular class room (upper right, upper left, lower left, upper left) which of these points are coplanar with the plane of the front wall.

Answer 1

(You copied 'upper left' twice, and you left out 'lower right'. 
But we know what you mean.)

If those are the corners of the wall, then they're ALL in the plane of
the wall (coplanar with it).