一个英语主谓一致+定语从句问题

问题描述:

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

先行词是老师 因为在用as well as 连用两个主语时 真正的主语是后面的teachers 所以先行词是teachers