I studied in the same school _as_ he did.和I studied in the s

I studied in the same school _as_ he did.和I studied in the same school that he studied in.是否都对
明明是同一所学校,为何第一句要用as 不用that,是因为所接的从句是省略句的缘故吗,第一句的先行词也是学校吗
我主要想问第一句的as明明是同一所学校,为何第一句要用as 不用that,是因为所接的从句是省略句的缘故吗,第一句的先行词也是学校吗.现在可以肯定第一句是对的,但我不明白明明同一所学校,为何用as