in a middle school 还是 in middle school

in a middle school 在一所中学
in middle school /in high school在中学

Does she teach English in a middle school? 主语是第三人称单数，将助动词does放到句首，动词teaches改成原形teach Does she teach English in a

都可以 视语境而定 若泛指在中学上学 则不加冠词 如 I studied French in middle school. 若特指某一中学 则加a 或者the，如 It is reported that a boy was shot in a middle school yesterday; Mike was the head of the student association in the middle school.

应该是：in a middle school 单数可数名词前通常要加冠词。

in a middle school 还是 in middle school
要看你在语句中的应用情况,若你要介绍自己曾经在某一所学校就学,又系句中首次提及,加不定冠词"a",若仅仅是泛指在中学,可以不加不定冠词"a".
例如：When I was twenlve years old I went to a middle school,in which I finished my school education.；In middle school,the students in China have to learn more scientific subjects,such as physics,chemistry,maths,and so on.第二句的"middle school"后面可以加"s",表示复数.
上述解答是否适合你问题的原意,