O'Connell's policy was to encourage the establishment of Catholic private schools outside the parochial system.

[3] The school opened with 21 ninth-graders and six teachers, all of whom were Catholic priests,[4] and charged $350 a year in tuition ($7,239 in March 2024 dollars).

[7] The foundation of the school reflected the increasing affluence of Boston's Irish Catholic community.

[17] In 2023, a consulting firm retained by the school disclosed that St. Sebastian's annual operating budget was $21.5 million.

In 2022, St. Sebastian's hired away Jon Bartlett, who had served as the athletic director at Boston College High School for the previous 21 years.

[21] The ice hockey team has produced 30 NHL draft picks, including first-rounders Joe Hulbig (1992, 13th overall), Rick DiPietro (2000, 1st overall), Mike Morris (2002, 27th overall), Brian Boyle (2003, 26th overall), Noah Hanifin (2015, 5th overall), and Will Smith (2023, 4th overall).