@Hollow7F
To be completely fair to the teacher (and her classmates and the bullies and her family and literally anybody else),
her incredibly poor reputation precedes her, and this complete 180 in only a few days (weeks?) in-universe is rightfully perceived as out-of-the-blue.
Also, this is just chapter
5.
If the teacher still acts this suspicious of FMC by, say, chapter 15, then yeah.