LOS ANGELES (SS) Katie Holmes has joined the PTA at her daughter’s new school! The actress, 33, has thrown herself into life as a single mother in New York with six-year-old Suri since splitting from Tom Cruise and is heavily involved in the youngster’s life at Avenues school. “Katie is embracing her new-found life as a member of the PTA,” a source said. “She’s getting involved in as many activities at the school as her schedule will allow.”