英文...唔識...
They are teaching in english language in all subjects.
They are using chinese language as the main teaching language.
Hope it helps you
圖片參考:http://hk.yimg.com/i/icon/16/3.gif
中文為教學語言= use Chinese as medium of instruction (CMI school)
英文為教學語言 = use English as medium of instruction (EMI school)
Your sentence is:
They use English as the medium of instruction.