They are located in Celestia. You must be level 48 to get to Celestia, and you have to finish the quest "The Final Countdown," (aka, the quest to defeat Malistaire Drake) They are located in the Base Camp there, and you must be level 50 to train the lowest one.
There are multiple star school trainers. The first of which is found in the main area to Celestia (not counting Dueling Diego). You will later find one in the Stellarium, then Zamunda. I don't know the rest, but hopefully that helps!