上课铃声应该改成哭了下课铃声应该改成泡沫
[shàng kè líng shēng yīng gāi găi chéng kū le xià kè líng shēng yīng gāi găi chéng pào mò]
These two sentences metaphorically suggest a negative feeling towards school life: class bell as 'cry' reflects pressure and dissatisfaction, dismissal bell as 'foam' expresses a wish for an end