一个英语主谓一致+定语从句问题

一个英语主谓一致+定语从句问题Every student as well as some teachers who is to visit the museum are asked to be at the school gate before 7.30 in the morning为什么第一处要用is第二处要用are?我知道第一处的主语是every student要用is可第二处不应该也用is吗?答案说先行词是teachers要用are,为什么teachers是先行词呢