Question

in a middle school 还是 in middle school

Answer 1

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，可以追问。

Answer 2

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.

Answer 3

应该是：in a middle school 单数可数名词前通常要加冠词。

Answer 4

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