Who is the teacher that (who) teaches very well in your school?
谁是你们学校教的很好的老师?
句中 that 或者 who 引导的是定语从句.这里用that 或who 作为关系代词都是可以的.但是前面已经有了一个 who,为避免重复,所以就得用 that 了.