What's the Cut Off for a School Year?

The word semphi is becoming popular and then becoming more recognized outside of japan. What desides sempi or cohi cas like ina, same age or same grade? They're not really sam there's no sanpica high relationship. So if you're one year older. I guess one grade higher. Thats sampi,. In the states, i guess the cut off for a school year is the end of, ok, stor what is it? Yi, think it's usually when the school year starts, so, like september.