主干是The classroom will soon be repaired.教室会马上被修理的.
中间部分是定语从句部分,其先行词是classroom,the door of which 即the door of the classroom,也就是说这个教室指的是门坏了的那个教室.
把句子分开看看：The classroom the door of which is broken will soon be repaired.
→The classroom will soon be repaired..
The door of the classroom is broken.
门坏了的那间教室很快会被修理的.