It was not obvious that it was a school;

是主语从句
句子结构是：
（It was not obvious that）（ it was a school）;
其实相当于后面的括号里“it was a school”这么件事“was not obvious”.
前面第一个It 指代的是后面“it was a school”这么件事,
说的是“它是所学校”这件事 “不太明显能让人看出来”.
但英文的习惯就是先说“那件事不太明显It was not obvious,”然后再说哪件事呢?that it was a school这件事.这个主语从句必须用that 连接后面的从句部分的,不然成了两句话,表达不同的意思了.“It was not obvious,it was a school;它不明显,它是所学校”.
例句：
However it is obvious that the elephant is not a column.
但明眼人都知道,大象根本就不是一根柱子.
It is obvious that this solution won't satisfy everyone.
很明显这种解答不能使每一个人满意.
类似的还有
（It is interesting） that （the three women have played starring Zheng Xiaoning's wife）.
有意思的是,这三位女主演都曾饰演郑晓宁的妻子.
是后面的括号里面的内容“有意思”
（It is surprising） that （a man like that was elected）.
那样的人竟当选真是令人惊讶.

that it was a school 是主语，第一个it是形式主语。
主语是一个从句的情况下通常会后置，再使用it作为形式主语。

属于主语从句，it是引导词，作形式主语

主语从句。首单词it 指代的内容。