I think Horimiya is what you are looking for.
Most of the descriptions in your question matches the details in the first chapter of the manga.
Synopsis (From MangaHelpers)
Hori may seem like a normal teenage girl, but she's a completely different person after school. In her workaholic parents' absence, Hori has been like a parent for her little brother since she was young herself. Between taking care of her brother, feeding them both, and housework, she doesn't have much time for a usual teenage social life. One day, she meets someone else who does not present his true self at school: a quiet, glasses-wearing boy called Miyamura. She'd assumed he was bookish, and possibly an otaku, but Hori couldn't have been more wrong. Outside of school, Miyamura is a friendly guy with many piercings, and he's not very good at academics. Now the two of them have someone with whom they can share both halves of their lives!
Here is the cover of the first volume, which shows both character in their "school mode". Note how Miyamura (the male character) leaves his hair long and has a nerdy look, while Hori (the female character) looks very stylish:
Here is a panel from chapter 1, showing Miyamura in his "after school mode" with plenty of piercing, which he hides by leaving his hair long:
Here is a panel from chapter 2, showing Miyamura's tattoos, which is the reason why he always wears an extra layer of clothes during PE to prevent the tattoos from showing through:
You might also want to check out Hori-san to Miyamura-kun written and illustrated by HERO, since it is the original work that Horimiya adapts from. The art is not as good, though, like the case with One Punch Man's original work by ONE (which looks like it is drawn with Paint) and its awesome adaptation.
