Sentence:  When caught for breaking the language rule, the punishment was to <WSD>stay back</WSD> in school and write lines of repentance for his wrongdoing: but in the school fields and canteens, Leow noted that, “students chattered in Mandarin and Hokkien as they wished with nobody to stop them.” 
Meaning of the word marked with <WSD> tags:  To remain (at work, school, organisation, country, etc.) after normal hours