Very sad. I'm sorry for your loss, and for the kids' loss. So have you taken all the eggs out now? Day 23 is not uncommon for some breeds, and maybe its just one or two eggs smelling?? I'm just seeing this thread, and was hoping for a better conclusion for you. But I have to ask, since you are a teacher... If you have taken the eggs out, did you open them up and perform an "eggtopsy"? I'm not sure if its appropriate for 11-year-olds, but it is an awesome learning experience. I would highly recommend it, for you at least, if you haven't already discarded the eggs. There are some great threads on here that can show you almost to the day, what day your chick died, by its growth.
Best wishes to you and your class.