he was nervous and lowered his head,BARELY looking up through his glasses at his students
barely是一个否定词,等于not.
如果改写你的句子,你就不难理解了：
he was nervous and lowered his head AND DIDN'T EVEN LOOK up through his glasses at his students.
(因为他紧张,所以他低下头.他不敢抬头看他的学生) .quickly在你的句子里面用不进去.