Well, sometimes schools in Japan end differently depending on the days. Usually they end in the evening, like around 5 or 6 because they have to clean the classroom. Then, on other days, maybe they end school earier because of a haft day ( or an make-up exam day thing.. like in Kare Kano. ^^; ) Or, I don't know, they must have a good reason for ending school at different times. *shrugs*
As for the rivialy between Meiling and Sakura, I like it OK. I mean, Meiling was just nasty to Sakura since she was seeking after the Clow Cards and Meiling wanted Syaoran to have them all. Despite her rudeness, Sakura still wanted to be friends with her. Same with Syaoran. They have been rivals, but all Sakura desired was that they be friends ( at least later on.. ^^;; )