Ah, 0verflow... Your game was so "revolutionary" back in the day... Well, it still technically is since no one has ever tried to do fully animated scenes like them. Not to mention this is what put Project Sekai on the map and on their way to going from fansubbers to official eroge translators.
Anyway, as someone who has finished everything this game had to offer back then and nowadays, I can give my full thoughts on this game.
School Days starts off innocently enough, putting you in the shoes of Makoto Itou, arguably one of the most infamous MCs of any VN title... and he comes from a rather... "unique" pedigree, given that the 0verflow story-verse is all kinds of fucked up, but that can be discussed later on once the other games are up for grabs here.
The story begins innocently enough, Makoto likes the innocent wallflower of a girl, Kotonoha, and this is picked up by his seatmate, Sekai. So Sekai tries to bring a guy with a rather flaky disposition and a shrinking violet together.
Seems innocent enough... until things get complicated.
Truth is, Sekai has a massive crush on Makoto, so she tried to do this to curry favor with him. How that whole "currying favor" with him goes ranges from teaching him how to be more straightforward to actually having sex with him under the pretense of "so your first time isn't awkward".
That's a major thing about the main trio. They all start off rather innocently enough, but as soon as they get into a relationship and experience sex, they either change for the better or go completely bananas. It doesn't help any that all of them have deep psychological issues that just never came up until they were thrust into situations that would force those emotions to crawl out.
And this is where player input plays the biggest part. Makoto, for as much as asshole he can turn out to be, also has all the qualities of a man that causes girls to fall for him because he never WAS an asshole. He hates bullies, he's a loving brother and while flaky, he is a good friend.
YOU, the player, decides whether he becomes a massive dickhole he is in the anime or learn to keep what's between your pants into just one girl. While he has moments of hesitation, even he realizes that he doesn't want to hurt the girl he (truly) likes in a good chunk of the played for straight endings.
But then again, this game gained infamy for the bad endings... Some of the most graphically shocking scenes (for their time of release) in an eroge, which shows just how dangerous these women are once they go off the deep end... Especially for Kotonoha, who is actually a very proficient swordsman (though that only gets revealed in Summer/Shiny Days).
Overall, I have rather fond memories of this game and I still have both the Japanese version and the Sekai Project version with me.
Also, Kotonoha is best waifu here.
PS: Almost forgot to mention, for anyone who likes the Highschool DxD anime? From the 1st season up to BorN, it was done by the head animator for School Days... who also did Summer Days... aaaaaaand Shiny Days... and even Cross Days... and even those cheap Rock Paper Scissor titles which had animated clips.