at a junior high school 和 in a junior high school 的区别

首先我觉得提问人的这个短语就是错的,没有junior high school 的说法,初中是middle school,高中是high school
at school和in a school的区别"
at school:表示某人正在学校,如:John isn't at home now,he's at school.
in a school:表示某人正处于上学的阶段,在学校学习或者在学校工作,如:He wworks(studys) in a school.