Make a component and copy that into the 4 corners. How I generally do it is to make it at an early stage. Draw a rectangle in one corner, double click on it (select face+bounding edges) then right click > make component. Make sure to replace selection with component (bottom left checkbox).
Now grab it at a corner with the Move tool and press Ctrl. This will create a copy rather than move the original. Finally double click on any of the components and start PushPulling to a length. Click somewhere outside to close the component.
Quickly, before the teacher gets back (did he go to smoke a cigarette?)