It's a spring day. Students are in class, but they're not complaining. They're working at five learning stations, with one station examining plankton, one examining lake sediment . . . And another steering the sailboat. For on this day, their classroom is the Inland Seas, a 77-foot steel schooner based in Sutton's Bay, near Traverse City, Mich. "It's a wonderful learning experience," said Barbara