This Is the Wizard's Secret Weapon, Vol. 2 (light novel) — Taro Hitsuji

Thanks to his unique skills, Rix has been allowed to stay at the academy. But despite Shino’s constant guidance, he’s been getting nothing but catastrophically bad grades, and he now faces expulsion for totally legitimate and “fair” reasons! As if things couldn’t get any worse, Tran, his sister-in-arms from his mercenary days, comes to the academy and plans to bring him back with her. But when she throws around her dragon powers on school grounds, Rix then faces…immediate expulsion?! In a last-ditch effort to not get kicked out, he tries turning Tran into his summoned creature—with shocking results!