Hi Andrea,
Sure wish my senior class trip went somewhere exotic like on a cruise and not to the local amusement park!
If you want memorable, book a private charter (since you have a larger enough group to make it cost effective) and take a trip to Sting Ray City Sand Bar. Getting kissed by a Sting Ray is not something easily forgotten, trust me!
Based on the size of your group, a catamaran that can hold the entire party might be the best bet. Red Sail has a few that hold 50-70 people, so I’d give them a try first. I’d start making inquiries now. You should also arrange for stops at Rum Point Beach, Starfish Beach and for more snorkeling, the Boulder Coral Garden off Rum Point. Make sure to bring a water proof camera.
Cheers,
Testudo