It seems that

有的语法学家(例如《英语语法札记》的作者薄冰教授等)认为 that 从句是主语从句，是全句的真实主语，而 it 是它的形式主语。但这种解释会遇到一个无法解决的困难：主语从句可以提前，而这里的 that 从句却是无论如何也不能提前的：

× That the headmaster would come to our class seemed.

比较：

It seemed strange to my brother that I liked physics. $\$rightarrow That I liked physics seemed strange to my brother.

这说明 seem 之后必须跟有主语补语(表语), 才能把 that 从句放到句首, 否则句子便不能成立。

因此,对于你所问的句子我们必须作另外的分析:此类句型中的 it 是无人称 it (impersonal it) 或虚设 it (expletive it),充当句子的主语。它只是填补句法上的空缺,并无实际含义。seem 在这里是连系动词,要求有主语补语(表语);that 从句便是它的主语补语。从信息上看,主语补语总是表示新信息,具有较大的信息载荷:

The blue dress is oldest. 这蓝色的衣服很旧。

The oldest dress is blue. 这很旧的衣服是蓝色的。

而在 It seems that 句型中, that 从句总是新信息的载体:

1. It seemed that everyone was satisfied. 好像每个人都很满意。

2. It seems (that) she was married and recently divorced. 她好像是结了婚,最近又离婚了。

3. It seems that he was late for the train. 他似乎没赶上那班火车。

4. It happened that the weather was exceptionally cold. 碰巧天气非常冷。

5. It appears that he followed my advice. (EED) 看来他听取了我的劝告。

6. It happened that I was not in London at the time. (EED) 碰巧那时我不在伦敦。