omitting 从 in 是。。。的 or something

[freefall]

It seems that sometimes 从 can be omitted from certain 是。。。的 sentences:

我是学校毕业的。

我是从学校毕业的。

我是北京来的。

我是从北京来的。

As far as I can tell, if the preposition isn't 从, it can't be omitted:

我是到北京去的。

*WRONG: 我是北京去的。

Can anyone explain this grammar phenomenon. Also, a native speaker told me that there is a slight difference between 我是北京来的 (I live in beijing) and 我是从北京来的 (I was visiting beijing). Is this simply a case of omitting 从? If so, can any other prepositions be omitted? Or is it some other grammatical phenomenon like topic-comment or something?

Thanks!

[againstwind]

I don't think there is any difference between 我是北京来的 (I live in beijing) and 我是从北京来的 (I was visiting beijing) , indeed. But the former one is an informal way.

It is probably an exception.