The moment was bittersweet when the 40-member cast of the Manhattan Theatre Lab HS’s rousing revue “What’s Next?” took their bows on May 1. Amid the cheers for the stars on stage, students, staff and parents in the audience shared a sense of sadness that this was the last act, the final curtain call for the Upper West Side school, which will be phased out in June. After 10 weeks of rehearsing six days a week, the 12th-graders predicted in song and dance what the years ahead would bring. Theater teacher Lisa-Erika James spoke with pride of the student playwrights, choreographers and composers, noting: “The arts are a powerful tool for developing leadership and promoting self-esteem, which will transfer into other areas of life for the students.” With the help of music teacher Matt Zimbelmann and paraprofessional and choreographer Sandra Dominguez, she said “the herculean task” created a special magic, which was obvious in the way it brought down the house.