班主任是容嬷嬷带来的卧底
[bān zhŭ rèn shì róng mā mā dài lái de wò dĭ]
In reference to a well-known TV series, 'Empresses in the Palace,' this playfully suggests that the homeroom teacher is an undercover agent sent by Grandma Nai (from the drama), known for her strictness. It humorously hints at perceiving the strict teacher as an imposing authority figure.