Inverts for a 20 reef?

I am setting up a 20 gallon mini-reef in my high school classroom. I teach aquatic science and zoology. I currently have a skunk cleaner shrimp, some snails and hermits, and a royal gramma. I also have two small frags of coral, just purchased- one is green star polyps and one is pink and not identified yet. The store called it Xenia but I don't think it's Xenia, as I've seen that before. I'm hoping to add some more soft corals, but are there any other inverts I could keep? I thought about a starfish, but I read that they may be detrimental to the sand bed and eat all the microfauna. I know anemones can be troublesome in tanks and the ethics of collecting them is somewhat questionable, so are there any other inverts I could consider?