校服一穿就变乖进了酒吧就变坏
[xiào fú yī chuān jiù biàn guāi jìn le jiŭ ba jiù biàn huài]
Translated roughly to 'Wear school uniform and become well-behaved; go to the bar and turn naughty.' The humor and social observation suggest the stark contrast between behavior at school (well-behaved in uniform) and after-school nightlife activities in bars (turns mischievous when freed from restraints).