This is based on opinion. Please don't list it on a work's trope example list.
Tearjerker / Chalet School
Although some fans consider this moment to be Narm-y, the moment where Rivals where Robin sings 'The Red Sarafan' to Joey, who is critically ill after being immersed in icy water while trying to rescue another girl, and pulls her back from the brink. Even Dick and Jem are struggling to keep it together.
In-universe, the Passion Play at Oberammergau in The Chalet School and Jo. Even the naughty American girls are moved to tears by it, and Joey is so overcome with emotion that she faints on returning to their hotel.
Joey's Heroic B.S.O.D. in Highland Twins, when she receives a black-edged telegram saying Jack has drowned at sea. Daisy and Robin are in bits, having never seen her like that before, and they beg Madge and Jem to come over and help. Madge is unnerved by Joey's eerie calmness and begs her to cry, but Joey can only laugh bitterly about 'the fortune of war'. Then Anna brings the triplets in - and Joey just breaks. It takes Jem giving her sleeping drugs to calm her down. Even after Joey finds out that Jack is alive, she's never really the same afterwards.
The death of Jacynth Hardy's aunt in Gay from China, and Jacynth's subsequent Heroic B.S.O.D., made even sadder by the letter which her aunt wrote for her before she went into hospital. She knew it was likely she was going to die and wanted to say goodbye to Jacynth.