So I am writing again to share my experience. I went to the Apple store and told the guy that I want to buy MB and Touch with that rebate, he asked me for some student ID so I showed him my Czech ISIC students card, he looked at it and found out that I don't study in US so he couldn't sell me what I wanted.
I decided to go to another Apple store and try my luck somewhere else. In that other store apple guy also asked me for students ID so i showed him again my ISIC, this time he didn't read much of the facts on it, he probably saw that it is a students card and that's all, so he sold me what i wanted.
Later that day I submitted rebate online and the rebate check came within a week.
So if you want to use that student discount you actually don't have to be a US student because Apple don't need to know any school/university info. If you shop online you are not even asked for Student ID or letter.
Glad to hear!