班主任正在上课你别捣蛋
[bān zhŭ rèn zhèng zài shàng kè nĭ bié dăo dàn]
This translates to 'Don't be naughty, the homeroom teacher is in class.' It evokes a scenario where someone is reminding a friend or implying a playful warning not to misbehave during an ongoing lecture.