I'm sorry for your loss!!
Like others have said some baby's just don't make it, even ones who were eating and doing great for the breeder may get stressed and take a turn for the worse in a new environment.
As terrible as it may sound this could be turned into a great class learning lesson. Some kids don't get to have pets in their homes and only get to learn about death and the grieving process when a friend or a family member passes away. Also I personally wouldn't rush to replace him right away either, as it may make some kids see pets as replaceable/expendable and if one pet dies you can just go get another. Maybe do a class poll to see when they think would be a good length of time to wait before getting another and if they can come up with some ideas to reduce stress for the new little guy/gal.