很高兴为你解答.
让我们还原到陈述句：This school is the one ( that )we are going to visit tomorrow .
这里用 the one 特指 the school ,做先行词,that充当定语从句里面的宾语成分可以省略.
如果是which ：This school is which we are going to visit tomorrow
这样的话,都没有先行词,你注意到没有!这样是错误的,先行词都没有,which 没办法来引导定语从句了.