Jack the Wicked (1995)

Blurb:
Servants of Hell such as wizards, witches and warlocks exist much closer to you than you may realize. Normally, they don’t shroud themselves in black, but rather live normal human lives while they ceaselessly wait for a chance to send demons into the human world. Like, for example, that quiet classmate who sits behind you … Naomi is studying for high school exams, but things aren’t going well, and due to her newborn little brother, she can’t concentrate, so she’s under a lot of stress. That’s where Jack comes in and introduces her to a demon named Nebiros. “He’ll lend you the power you need to pass, but in return, you need to pay him a soul.” At first, Naomi refuses to make the deal, but then, Jack whispers to her. “You can give up your little brother instead …”

Source: www.anisearch.com/manga/65308