Donald is right. The cardboard needs to be against the ground. Then cover. Plastic or landscape fabric (double layer) and weight that down with rocks or any thing you have. You are not going to leave the plastic or landscape fabric permanently. Just an inexpensive way to help block sunlight and water. Sometimes I have to add more cardboard and sometimes make multiple layers of every thing. You may have to cut the cardboard into pieces to fit it in flat. Good luck and joy to you! May there always be a cloud over you as you work outside.
Also, if you can stress the weeds first, weed whacker and pour boiling water over. I think chemicals may be a no-no being school property.
