Because they're Hentai and 'connected', this is a review of this one, it's prequel and it's sequel, they're all adapted from the Visual Novel "Sei Yariman Gakuen Enkou Nikki" by Orcsoft Team Goblin and animated by PinkPineapple
The 'story' is simple, there are two scenes, in each one of them a different prostitute and a different client do their thing, the end. xD
The prequel has the same 'story' but has a gangbang in the end.
The sequel is very different but just as simple, it's just a guy and 2 of his cousins fighting for who takes his virginity.
So yeah, there's no continuous story you
have to follow, no need to watch the others.
In terms of overall quality, Shin Sei Yariman Gakuen Enkou Nikki The Animation improves A LOT.
The animation is amazing, you can see detail even through the censorship, there's no repetition and the characters do more than just the normal "back and forth".
In the prequel the animation is exaggerated, the characters move a lot but there's no fluidity, it's jumpy and seems to be missing frames.
The designs are good because of the source VN but they added so much shine to them.
For some reason, in the third hentai they went back to this kind of animationn.
That is the reason I only recommend this one.
There's really nothing wrong with the others, the animation is still above average and at least they're not full of rape, netorare and the like.