| Quote #1
One of Miss Minchin's chief secret annoyances was that she did not speak French herself, and was desirous of concealing the irritating fact. (2.24)
This is the moment when Miss Minchin really starts to dislike Sara—because she has a desired skill (that pretty, fluent French-speaking!) that Miss Minchin doesn't have. Notice how the jealousy is way more about Miss Minchin's inadequacies than what Sara actually has?
| Quote #2
Lavinia, in fact, was spiteful. She was inordinately jealous of Sara. Until the new pupil's arrival, she had felt herself the leader in the school. (4.6)
Your days are numbered, Lavinia. Next time, try being nice to people. Seems to work for Sara.
| Quote #3
"She's ridiculous without being rich," Lavinia sniffed. (6.4)
Lavinia can't find enough bad things to say about Sara, even though she hardly knows her. Check out the fun way that Burnett makes Lavinia "sniff." Can't you just see her little nose stuck up in the air?