This online manga was about a boy who had to summon a monster for something at his school and finds a book and summons a demon. She easily defeated multiple people and their monsters and that's pretty all I can remember other than that the demon the boy summoned was hot and I read it on a website not any apps if that helps, I don’t remember the website name.
Might this be Magika Swordsman and Summoner?
15 years ago, the world discovered magic as well as mythical beings from another dimension. Humankind learned to summon these dangerous mythical beings and used their power to great effect. Seven countries formed contracts with these "gods" and gained their allegiance as a result. Summoners on their 14th birthdays will be chosen by a God and given a stigma. Schools have been set up to train summoners and magical swordsmen as a result.
15 years later after the finding of magic, Kazuki Hayashizaki is chosen as a summoner and given a stigma to indicate that. It is surprising because males aren't chosen as summoners due to their low amount of mana compared to females. As the first male to become a summoner, Kazuki is under a lot of scrutiny by almost everyone.
His "diva" summon is Lemegeton.
Leme is a petite young girl who has long silver hair, dark tan colored skin, yellow eyes as well as white star-shaped tattoos on her forehead, arms and legs, Leme is always seen with a white tunic and black armbands. When Leme transforms to her original form, she becomes noticeably taller and her breasts become bigger, in this form she wears a revealing black dress with yellow bracelets. The marking on her forehead becomes bigger and a black "halo" appears behind her.
Found with a search for
manga school fight "summon * demon"