We are talking about a sailing vessel here, not an aircraft. With a window of six days, i.e. three days out and three days back, assuming you don't stop more than an hour or two at your destination, then you will not get very far at all.
Sorry to disappoint you but I can't see anyone taking along three students for six days and especially not for a quick there-and-back jaunt to Cuba, an island not often visited by cruisers and fraught with administrative potholes.
Aye // Stephen