Historical clue formulations

• Where high schoolers work on their numbers? — 2025-12-14
• Campus singing group — 2023-05-22
• School group working in harmony? — 2016-04-09
• School singing group — 2015-01-13
• Harvard has an all-male one — 2013-12-07
• Extracurricular activity traditionally for men — 2007-06-09
• Happy chorus? — 1999-03-17
• Group in robes — 1996-12-22
• Group working in harmony? — 1996-08-29