l have taught English in the middle school

问题补充：

l have taught English in the middle school

答案：

l have taught English in the middle school since 1980

这个句子是现在完成时,用in ,after的话前面就应该是 i taught.

如果用for的话,后面就应该接一段时间,例如3 years,很显然1980不是一段时间.