Is it wrong to write "god" instead of "God"?

I remember when I was a child I wrote a word “god” in my exam and my teacher got angry for this one,stated that it’s wrong for disrespecting the ‘higher being’,and that kind of “God” word need special treatment in use. I know that in most language/for most people the first letter of “god” must appear in capital letter no matter where we put that word. I think this is silly,is it because many people afraid of the word “god” that will upset their God? Or is it because many people just use the common rule of writing this particular word? I have no problem since I’m an atheist now but it’s silly if I upset other people just because of this.