We have done this in many of the classes I have taught, usually while we are waiting for paint to dry or some such.
A rectangular shaft of cedar/spruce/pine/cypress/fir, or whatever you have, can be shaped to an oval, and the ends cut in an arc. 4mm ply blades are then glued and nailed or screwed to these arcs and, with a little sanding, a very functional, inexpensive and fairly light paddle can be had.
You can cut the shafts and blades ahead of time, or let the students do it themselves if there is time during the class.

Good luck!