What is difference between 'back in high school~' and 'when I was high scool~'?
If I want to say something during the high school, which sentense is more common use? For example, 'When I was high school, I studied English hard ' or 'Back in high school, I studied English hard '

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well... whatever  your mind its important

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"When I was in high school" is correct
The first example ("When I was high school") is not correct English.

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Both are very common.  Which I would use would depend on how I was feeling.