Ughggh I just read this whole thing in an hour and now I feel like this mangaka reached out a metaphorical hand from 2013 and stole that time from me.
This manga tried to be shocking, but did it in the most base, least original, and most infuriating ways possible.
Make your readers feel something other than irritation at the sheer stupidity of the plot.
I need a palette cleanser after this. I think I gotta re-read a few chapters of Ore wa Lolicon ja nai! before I go to sleep.
If anyone wants to read a GOOD manga where a failed 30 year old pedo perv dies and is reincarnated into his past self, go read that instead. That's ecchi comedy, not psychological horror, but the initial setup (our MC visits his old school, something happens, then voila, reincarnation!) and overall premise is nearly identical to this one, so I thought it was worth mentioning.
edit: thinking of the wrong manga. I mistakenly thought it was Ore wa Lolicon but it's not. There's a manga where an adult revisits his old school, somehow falls into the pool and maybe drowns, but is reincarnated as his younger self. At occasional points in the story his perspective changes back and forth between his adult and child self.
edit2: The manga is Mujaki no Rakuen, by Uran, the same mangaka who did Ore wa Lolicon, so of course I had them confused