These are high school DxD that I have read, and highly recommend it. Grammar are pretty good. Interesting plots.

DxD:Multiplayers

By-SpatialPhoenix

FanFiction.net

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A Turk in DxD Game

By-TurkishBoy

FanFiction.net

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Timeless Conquest

By- Rain Sennin

FanFiction.net

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A Hero Journey

By-HolyKnightsofTheRoundTable

FanFiction.net

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Dxd:Codex of the Universe

By- flameclawsxxx

FanFiction.net

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HSDxD: The Power of Wildcard

By- Authorian

FanFiction.net