It's （a so interesting topic） that all students joined the d

这里牵扯到so和such的用法.应该为 so interesting a topic
1、so + 形容词/副词 + that + 从句,如：
This story is so interesting that I want to read it again.（这个故事如此有趣以至我想再读一次.）
He spoke so quickly that I couldn't follow him.
（他说得如此快以至我不能跟上他.）
2、so+形容词/副词+(a/an)+(单数)复数可数名词／不可数名词+that+从句,如果句中的名词是单数可数名词,其前就要用不定冠词a或an,如果是复数可数名词或者不可数名词,前面就不用冠词,如：
She is so lovely a girl that everyone loves he（这个故事如此有趣以至我想再读一次.）r.
（她是如此可爱的一个女孩,以至每个人都喜欢她.）
Those are so beautiful flowers that the girl wants to pick them.（那些花是如此漂亮以至那个女孩想要摘下它们.）
3、such+a/an+形容词+单数可数名词+that+从句,如：
She is such a lovely girl that everyone loves her.（她是如此可爱的一个女孩,以至每个人都喜欢她.）
It is such an interesting story that I want to read it again.（这是一个如此有趣的故事以至我想再读一次.）
4、such+复数可数名词/不可数名词+that+从句,如：
He showed such concern that people took him to be a relative.（他表现得如此关心以致于人们都把他当作亲戚了.）