Question

He said,"I was late for school." 间接引语是He said that he was late for school.为什么不用过去完成时

Answer 1

陈述句的间接引语，过去时间推移到过去的过去，要注意：
当强调动作或状态先于引述动词时，一般过去时要变为过去完成时。如：
He said, "I didn't recognize you." 要变为 He said he hadn't recognized me.
但当“过去的过去”已经不言自明，或者通过其他词汇手段予以体现，或者侧重于强调所转述的事实本身，一般过去时形式可以不变。如：
Ann said, "She emigrated to Paris in 1956." 要变为 Ann said she emigrated to Paris in 1956.

直接引语变间接引语其实并没有那么简单，还有很多其他特殊情况，在此也说不完整，建议你自己看一看语法书。
希望对你有所帮助。