There's also 'Percy and Annabeth: Middle School Years'. This fanfiction delves into how they first met in middle school. It shows their initial misunderstandings and how they gradually became friends. The author does a wonderful job of portraying their characters as they would be in middle school, with Percy's impulsiveness and Annabeth's intelligence.
Another good one is 'The Middle School of Percy and Annabeth'. It tells the story of their middle school experiences through different events like school projects and competitions. It's really interesting to see how their relationship develops in a middle school setting, and how they face the typical middle school challenges together.