They have more in common than it seems :
- The vibe, despite the setting being in high school.
- The deeper meaning.
- The extravagant cinematography.
- Scandalous female teacher.
Hierarchy's characters are not villains like in 7 Escape but there are similarities.

Taboo, intimate, focused on adolescence. Both cover issues that adults consider trivial and are stories of "raging hormones gone wrong." (Aku no Hana being worse in every way) If you relate, you may find some solace as well.

"By the way, there’s the drama Shounen no Abyss I need to watch, which also picks up on the idea of a teenager trapped in a rural town (the difference being that he doesn't have an "ordinary" life like Kasuga in Aku no Hana). A friend who’s a fan of J-dramas recommended it to me, and it seems really dark, completely unconventional as well." From my review of Aku no Hana: https://mydramalist.com/profile/suwunoh/reviews/399280

I can't believe no one made the comparison. Even the titles are enough. If you loved 7 Escape, especially the survival part on the deserted island, you will love Missing 9. Missing 9 feels even more random than 7 Escape too so if you like that then it's perfect.