The person to who you spoke is a student of Grade Two.为什么不能用

本句是引导词做介词to的宾语,是要用whom的.
如果先行词在定语从句中做宾语而且指的是人的话,那么必须用whom/that,不能用who.不知道你说的who可以代宾语是什么情况?
如：
He is the person whom we visited yesterday.
从句中：we visited (the person=whom) yesterday.先行词是做宾语的,所以只能用whom/that
这种情况可以用that,并且that可以省略,变成He is the person we visited yesterday.
但是你题目中这种介词在前的就只能用whom不能用that.
再问： She married the man who we met yesterday.这句话中who 不是代宾语么
再答： 这句话从语法上说是不对的，应该用whom。