Leah
I once gave a class on the subject of Pilegesh (and outside of marriage relations) which was highlighted by a controversial responsa of Rabbi Yaakov Emden. שו"ת שאילת יעבץ חלק ב סימן טו
I can send you the source sheet if you like (let me know - avbillet@gmail.com).
Rabbi Emden's efforts will give you all the information you need, as well as his startling (thought widely unaccepted) conclusion.
I don't know the age of your students, and it is likely not the best choice to go through with them, but you will likely find it to be enlightening for yourself as you filter appropriately.
Good luck
Avi Billet
