Celebrity as a Cruise Line is wonderful and very upscale compared to RCCL. As far as the Xpedition, although it is Celebrity, it is not comparable to the Luxury Liners due to its itinerary, size and passenger capacity. It seems to me that it is inappropriate for High Schoolers since most passengers liked the low keyed relaxation after returning from the days expeditions. Many of the passengers were retired couples and the average age was about 55 to 65.
There are many other vessels which are very casual which would be more suitable for your age group. They wouldn't require dressing for dinner either as we did.