班主任毕业时才明白容嬷嬷结局时才觉悟
[bān zhŭ rèn bì yè shí cái míng bái róng mā mā jié jú shí cái jué wù]
'Rong mom' refers to a famous TV series character from 'The Story of Minglan' (or other Chinese historical dramas where this specific role appears) - an elderly woman always supporting or advising. The nickname humorously implies that only at graduation (for students) or at Rong mom's ending scene in the drama (for watchers), people gain some realization or sudden understanding about the school days or story plot, respectively.