Dozens of bathers were evacuated Saturday from a beach at Nice on the French Riviera after the discovery of an explosive shell probably dating from World War II. The explosive shell on a beach along Nice's famous waterfront promenade was discovered by a teacher in charge of a group of handicapped youths who was checking out the bathing area before the group went swimming. He brought the metal shell to a help station on the beach which gave the alert. Several fire engines and an ambulance rushed to the scene while bathers were evacuated from the popular beach. Fire officials said there appeared to be no danger, but a security perimeter was set up for several hours while officials waited for experts from Toulon to neutralize the device.