A school administrator who helped transform a low-performing Worcester high school into a model of success was named national principal of the year Tuesday, a first for a Massachusetts educator. Sheila Harrity, principal of Worcester Technical High School, was among three finalists for the honor. She received the award from the National Association of Secondary School Principals and MetLife during a surprise assembly at the school. “I really view this as a community award,” Harrity said by phone afterward. “No one person could do this all alone.” The Worcester native will be honored at a black-tie gala Sept. 20 in Washington, D.C.