额..我是不是可以这样理解,你的 ---- 是下划线的意思.
如果是这样,那就是 He ran back to my class room which he had left his books in.
这个是which作定语从句,因为left books和class room做连接,所以介词用in.
如果下划线不是限制只能填一个单词的,或者可以不填的情况：
也可是 He ran back to my class room in which he had left his books.
这里的in which是一个倒装的用法,一般是从句特别长的时候会用.
或者是 He ran back to my class room where he had left his books.
注意where作连接的话句子末尾就不要加介词了.
顺便说一句,是不是你的题目分类选错了...