The two rings prominent in the movie feature a circular stone and a cross respectively. When linked together they form a well known female gender symbol, echoing the themes at the center of the movie and foreshadowing the later discovery.
The safe of AD1 includes files on or pictures of various famous women: Sylvia Plath (marked terminated), Indira Gandhi, Jacqueline Kennedy (as Jackie Onassis, terminated), Rosa Parks, Janis Joplin, Mother Teresa (terminated), Princess Margaret (terminated), Katharine Hepburn, Margaret Thatcher (neutralized) and Marilyn Monroe (terminated).
At the beginning of the film the sailors are heard singing "Fifteen men on a dead man's chest / Yo ho ho and a bottle of rum!" This is not a genuine sea shanty but was created by Robert Louis Stevenson for the novel "Treasure Island", published in 1883, so the sailors would not know it.
When the girls are at the school looking for the ring, the headmaster goes into his office and the painting on the wall is in perfect condition while all the girls rush out of the office secretly. Then, when they are in the van heading back to St. Trinian's, it shows that they ripped the painting to get the ring.