1.This school is the one where I spent my childhood.
这个句子中,is后面的内容是表语,如果没有the one 这个词,句子不完整,the one 在这里既当主句的表语又当从句的先行词.
2.This is the school where I spent my childhood.
这个句子中,This is the school已经是个完整的句子,school是从句的先行词.
3.This school is the one which/that I visit.
4.This is the school which/that I visit.
这两个句子中,school和the one在从句中当宾语（which/that）,要区别于上面两个句子中这两者当状语（where）的情况.理由同上.