Boku no Ushiro ni Majo ga Iru

When Ryuusuke Nanri changes schools after his parents' divorce, he is made to sit in front of a strange girl called Miya Fujimoto. Miya always wears Gothic Lolita-like fashion, carries a strange book and her cat Judah around with her and claims to be a witch. From now on, Ryuusuke will become the main target for all her strange witch experiments...

(Source: MangaHelpers)