Based on the erotic game by Tinker Bell.

Which anime start bad but gets amazing afterwards?

I hate ecchi trash anime, but Prison School surprised me. So it starts with a character who looks like this (She usually wears a revealing dominatrix outfit but don't want to ruin anyone's innocence). Squishing these five boys with her breasts and bum as punishment.
The episode further goes on to later itself with grotesque fanservice and quickly reveal the five boys (the only males in a previously all-time boarding school) to be raging perverts who try to peep on the girls locker room. But what seems like a show written purely to get teenage boys rock hard is actually a surprising well written comedy/drama. Jokes are rarely one-off gags used for a cheap laugh.
Instead a joke will be introduced early on, and over time the anime actually expands on it as much as possible.
And while there are many tits-in-face or crushing-guys-nuts jokes, the raunchy dirty humor is surprisingly creative and original.
But what caught me off guard the most was how these jokes affect the characters. One of the most recurring feelings these teens experience is embarrassment.
After all, it's not easy being that age. Oftentimes in all entertainment, someone will do something embarrassing and just move on with their lives (see any episode of the Office.) But the shit that happens in Prison School genuinely affects them as people, and these gags actually lead to further plot developments further down the road.
This blew my mind as I slowly fell in love with the anime. This anime also has one of the best casts I've ever seen.
With a few exceptions, the main cast is either well fleshed out or incredibly memorable.
Often both.
There is also some surpingly endearing romance and friendship. So yeah this garbage looking pile turned out to be one of my favorites of all time. Thank you Emperor Lelouch for your infinite generosity in requesting my answer.