校服一脱就是一辈子
[xiào fú yī tuō jiù shì yī bèi zi]
'Once the school uniform is taken off, it's a lifetime.' It's a metaphorical phrase used to suggest that the moment one steps out of school and adolescence (represented by taking off the school uniform), they step into an adulthood where everything has changed permanently, symbolizing an important and irreversible transition in life.