Witch Satsuryuu Seminar ~Tensei Kenja wa Majo no Gakuen de Ryuu Koroshi o Mezasu~

A boy who trained obsessively on modern Earth to become a dragon slayer is reborn in another world, where he masters the ultimate dragon-killing magic under a legendary sage. Now known as Jeff and wielding the strongest power imaginable, he heads to the kingdom's top academy, following his mentor's dying wish to use his abilities for good. But dragons have long been extinct, and the school turns out to be an all-girls witch training institute.