Shaggy calls Coolsville High, "[the gang's] alma mater".
The whole gang attended this school and graduated from there (though some attended otherschools along the way). Shaggy calls Coolsville High, "[their] alma mater," with the plural implying the whole gang.
It is never stated that the Devils are playing on their home field, but they must be or Melvin Gerkin wouldn't have access to mess with the scoreboard.
References
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