A nationwide trend toward declining enrollments, combined with the higher cost of salaries as the number of teaching sisters fell, brought St. Thomas through a series of challenges throughout the 1970s.

The convent, named Notre Dame Hall, was converted to provide additional classroom space and to house the Guidance Department, which includes offices, a library, and a conference area.

Teams of note include football, soccer, swimming and diving, ice hockey, basketball, lacrosse, tennis, track and field, alpine skiing, cross country, and baseball.

Tennis player Julia Keenan became the first high school athlete in New Hampshire to win four straight individual titles in any sport.

Among St. Thomas Aquinas' graduates are Ron Fortier, comic book and pulp writer best known for his work on The Green Hornet,[citation needed] Brendan DuBois, author of numerous novels including Resurrection Day,[2] Olympic runner Rachel Schneider,[citation needed] and Baltimore Orioles baseball player Ryan McKenna.