What from of sentence you study in the beginning
 

I heard that some book or school teach simple form of sentence first then go to more polite form later (like いく > いきます、　いかない > いきません)

but for me I start learning the opposite (いきます　＞　いく, いきました > いった)

How about you? and is it different in the end?

I learned polite first. I think it's a very stupid way of teaching Japanese to students you are trying to make fluent because it takes a very illogical approach to grammar.

If your students are just trying to learn vacation Japanese, then it's smart.

I also learned the polite forms first. My first text book used this approach.