This guide contains spoilers to those who HAVEN'T played the game, you have been warned!
In this guide, I'm going to tell you how to have special "alone time" with certain characters, in which you and another boy will get naked and masturbate with each other.
To have these moments with a certain character, you'll need to build up the Friendship rating of that character, and to do so, you'll need to hang out with them often and watch Space Sheriff with them.
You can pick the following characters, these characters are just learning how to play with their 'wee-wee', you have to help them learn how to masturbate:
The first two, Tsutomu and Kotarou are tricky to build up Friendship ratings with, but here's the secret of how to do it. Hideaki and Genta, their niichans (big brothers), are always out on Saturdays, so you go their house on any Saturdays and interact with either Tsutomu or Kotarou, that'll build up their Friendship rating and then on May 2nd, go to their house and you'll teach one of them to play with their wee-wee and masturbate.
The latter two, Yuuta and Tomoyuki, they can be visited on any day of the week as soon as you've met them. Build up the Friendship rating with them and then go to their apartments on May 2nd to help them masturbate
On May 21st, after the victory celebration over Genta winning the school cook-off contest, during the night, go visit Genta at his house. He will invite you inside to his room, and then tells you his problems and you help him masturbate
It's morning and you are visited at your house by Shugo, Hideaki or Chiaki, depending on which of them has the highest Friendship ratings. You exchange a few words, get naked and then masturbate each other.
Was this guide useful? I hope so! As far as I know, there are no other characters that you can get naked with and masturbate with.