Others use a projector or some other type of directed light source. With a simple maglight or some wide beam source, it might be harder to see though the eggs. Unless you have a bright directed light and light colored or white eggs, you often won't see as much, thus my recommendation that new hatchers don't toss eggs till day 13-14 or so.
That said, white eggs are MUCH easier to candle:
(Taken with the same camera, but using the surfire6p on day 12 or so of a white egg)
Super Cool! I sure could have used this a week ago. I've searched all over the internet and there aren't any great example like yours! Here is a link that shows what my class has been working on. I posted pics of the incubator and eggs we are hatching. We are on day 12 today. So far we have 19 out of 24 that are ok.
It's often hard to find good sequential pics of egg candling so that's why I decided to make a thread out of this batch of chicks. It's going to be updated as the eggs incubate. Since you are teaching a class, might want to take a peek at the paper "A series of normal stages in the development of the chick embryo" by Hambuger and Hamilton from 1951. It's really old, but is really detailed on the process of development.
Best of luck to your class project! I've always had issues with classroom hatches due to temp fluctuation on weekends and classroom bators not keeping up.