HAPPY ENDING: There were long faces all around when the students at Van Nuys High were told that they couldn't perform "Joseph and the Amazing Technicolor Dreamcoat" because it would compete with a "Joseph" revival at the Pantages Theater in Hollywood. . . . But the licensing company offered an alternative: the Tony Award-winning musical "Once on This Island." Columnist Scott Harris took in a performance and gave it a rave review (B1).