How to make a red reveal message
1) Use Photoshop to create a distracting pattern with red, magenta, yellow on different layers. Patterns can be rows of nonsense letters, textures or repeating patterns. Try to make these coincide with the position of the message lines so that it obscures the message. The crazier the patterns the better.
2) Add the message layer below these in cyan color, this will appear black when viewed through the red filter. Make it 50% transparent so that it is less obvious against the other layers.
3) Use the Multiply Blend on the upper masking layers so that it blends with the cyan beneath and not just cut it off.
You could also print out the hiding layers by themselves have have students draw their messages with a cyan magic marker.