Thomas College in Waterville will have the largest incoming class in its 122-year history this fall and plans a new dormitory in the near future to handle the increasing number, and Thomas is just one of several central Maine colleges that have more students, new administrators or new programs as the academic year begins. The private college welcomes 320 new students this weekend in addition to its 550 returning students who hail from 15 states and seven countries, a spokesman said Friday.