When Miss Clavel and the girls are outside Madeline's hospital room, Miss Clavel says "it's only been a few days since the operation". When they're about to leave, Miss Clavel says "Madeline's had a hectic and sleepless evening", implying that they are visiting her the day after her surgery. Miss Clavel was clearly trying to cut the girls' visit short.
Madeline stands up on her hospital bed to show off her appendectomy scar. Her IV line is very short, so standing would be very difficult, if not impossible.
At the end of the film the song "What a Wonderful World" by Louis Armstrong plays. However, the film is set in 1956, eleven years before this song was released (1967).
The Spanish Diplomat rides around in a 1961 Vanden Plas Princess 4-litre Limousine. This movie is set in 1956.
At one point, Serena says to Chantal "You only just found out how to tie your shoelaces." The girls wear Mary Jane shoes with straps, not lace-up shoes, throughout the film.
When Miss Clavel and the girls (except Madeline) are leaving the carnival, they depart in a small bus driven by Helene. It is never revealed who the owner of the bus is, but it may belong to the school. Furthermore, the exterior of the bus is never shown.
When Lord Covington enters Lady Covington's hospital room, he asks Madeline "Who gave you permission to be in here?" Madeline replies that nobody did. But when she first peeks into the room and says hello, Lady Covington does allow her in.