其实这就是个that从句,本身是有that,指代the one(one就是学校) 因为是作为visit的宾语,that是可以省略的.
Is this school the one （that） you visited yesterday?
2.跟上面的是一样的说法,定语从句中的that指代的东西作为动词宾语时都可以省略的.
补充：其上上面两个句子你可以写成陈述句来理解的
This school is the one you visited yesterday
This is the school you visited yesterday
定语从句一定要特指某个名词进行修饰的