Most rainbow demos in the classroom are actually NOT rainbows, but merely another example of dispersion. EVERYONE has seen light disperse through a triangular prism. Unfortunately, this is NOT a rainbow.
You can reproduce, using regular science class stuff, a real rainbow in your class. Fill a relatively clean large, 1 liter works well, beaker with relatively clean water. (Both clean beakers and water are hard to find around here, but...). If you have a good bright white light source, use it. Otherwise, procure an elder projector. I use an old slide projector. Aim the projector at the "EDGE" of the beaker, just so the light grazes the beaker. Adjust as needed. The optimal angle of incidence is 42 degrees, which is achieved since the "side" of the beaker is round. Turn out the lights. Behind you will appear a brilliant rainbow. This is exactly how a rainbow is formed - thru a combination of refraction and reflection in the water drops. Notice, the rainbow is vertical. This is because of the vertical shape of the beaker. For a circular rainbow, I use a very round flower vase I subversively borrowed from my Mother-In-Law. (I'll return it when I'm done.....teaching....)