I hear our teacher will be back__three week's time

不是 at
正确的介词是 in
back in three weeks' time将在3 个星期内回到这里
句子可能是
I hear our teacher will be back at school in three weeks' time
写错了.为什么？没有为什么的in three weeks 是固定搭配，人家都是这样说的。我是加拿大人，英语是我第一语言。