首先把这句话改成陈述句,这样我们能看的更明白些：
This school is ______ you visited the other day.
显然,在is之后缺一个表语,必须补齐,即：the one.
如果用that,则that前必须有一个先行词,即the one.全句是：
This school is the one that you visited the other day.
因为that 在定语从句中做宾语,因此,可以省略.