Soup, guess who is here with a headcanon about delinquents? That's right, me!
I read somewhere that the guys chose their t-shirts to match the color of their eyes. (I can't find the screenshot...) And also, the color of their eyes is their favorite color (I won't comment on this part...)
But I have doubts that they would be able to find t-shirts with designs that reflect their hobbies (Hayanari's handcuffs with a key - he's interested in locks and wants to create his own, Hokuto's gun - he's curious about the firing mechanism, etc.)
My headcanon is that they drew it themselves.
Hayanari, Hokuto, and Gaku have a positive attitude towards art, so I assumed that they might be interested in sketching, graphic drawing, graffiti, and so on.
Now, imagine that the guys actually bought t-shirts at a local store, but they gathered at Gaku's house and started creating art there. Dairoku doesn't like this activity (another headcanon, he's left-handed and struggles with a pen, has messy handwriting), so he explained what he wanted to see and simply observed the proceedings. Umeji isn't a pro either, so he just drew a small sketch and handed it over to the trio.
As a result, Hayanari, Hokuto, and Gaku spent the evening drawing on T-shirts.
Why Gaku's house? He and Umeji are the only ones with a positive family conversation topic. But Umeji has several siblings, so it would be very noisy at his house, and Gaku is an only child, so his house was the only option. Gaku's parents, on the other hand, are happy that their son has friends with whom he can try out a new style (they witnessed their "experience" with appearance changes, but didn't intervene).
But it doesn't mean that only art trio was busy back then. While the guys were drawing, Dairoku and Umeji bleached their own hair and dyed the hair of the others. (I remind you, they dyed their hair from their anime colors to blonde to copy Osoro)
I just don't understand where they got their weapon bags from. Did they steal them from the sports club? These bags come with book bags, so all students get two bags? I don't know...
By the way, if you didn't know, where are their scars.
This screenshot I took from the wiki, and I still don't support YandereDev.