Comments about Arbor Scientific Slide Mounted Polarizing Filters 35mm 50/pk:

These polarizer slides are inexpensive enough that you can purchase classroom quantities. Order enough to hand out three slides to each pair of students in the class. They can manipulate the slides themselves to see how light is blocked or transmitted, depending on orientation of the slide. Then have them investigate how they can "unblock" the light already blocked by two slides by inserting a third slide. (Most students want an explanation, so look up Malus effect in advance.)