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,