Some Tumblr blogs are dedicated to 'The 100' fandom and might have links or even host their own bellarke high school fanfics. You can search for relevant tags like #bellarkehighschoolfanfic on Tumblr to find these blogs.
You can start by looking on Archive of Our Own (AO3). It has a vast collection of fanfics, and there are likely many bellarke high school ones there. Just use the search function and type in 'bellarke high school fanfic'.
Chapter 576, "The Electronic Pickles are Gone!", is the clearest early answer because it turns Ai Xiaochun's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Tonight, Team Q exceptionally didn't cook.
Chapter 731, "Ye Chu Died", is the clearest early answer because it turns Bai Lu's situation into a concrete plot event. It opens with Dean, can I really not do it? and closes on She'd been acting off these days.. The key payoff is that the chapter closes with "Why did you come to the sanatorium?
Chapter 707, "I am Daoist Qingfeng", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. The chapter turns that into something concrete by moving from Su Yinwan hadn't finished her dessert but, feeling agitated, had...
Chapter 797, "Bu Fan and Bu Chen", is the clearest early answer because it turns Bu Fan's situation into a concrete plot event. It opens with The night grew deeper, and while waiting for news, Jiang Li dragged...
Chapter 703, "Slap Oneself", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. It opens with Wu Daoshan quietly raised his head to size up Zhao Lang, and sure... and closes on Young Master Gu was intending to back Jiang Li..
Chapter 595, "Four-legged Black Toad", is the clearest early answer because it turns Four-legged Black Toad's situation into a concrete plot event. It opens with Jiang Li steadied himself with support, endured for a while, and then...
Chapter 841, "Isn't this Daniu?", is the clearest early answer because it turns Aunt Zhao's situation into a concrete plot event. The chapter gives readers a real answer by opening on Zhao Lang took the photograph and glanced at it, chuckling, "It...
Chapter 803, "Younger Brother Misses Mom", is the clearest early answer because it turns Mom's situation into a concrete plot event. It opens with Brother, do you think hiding here is a bad idea?" "Don't worry,... and closes on It was as if she had been replaced by a different person..
Chapter 837, "Why did she come?!", is the clearest early answer because it turns Rong Ya's situation into a concrete plot event.
Chapter 528, "Cursed", is the clearest early answer because it turns Shang Shaojing's situation into a concrete plot event.