The teacher let out a tsk-tsk when she saw the student playing with his phone in class.

tsk-tsk 是拟声词，模仿人们用舌头发出"啧啧"声的声音，通常用来表示不赞同、失望或责备的情绪。这种声音是通过舌头抵住上颚快速弹开发出的。