首先teach 和 is 都是动词,同一个句子中不能有两个动词.其次你说的she is teaches in a middle school in beijing.有错误,应该是shi is a teacher in a middle school in beijing.
如果使用is的话,你所说的两句话,在句意上就有点区别.she teaches in a middle school in beijing.强调的是teach.如果用is.she is a teacher in a middle school in beijing.强调的就是shi is a teacher.具体要用什么,你需要根据上下文来确认.