There's a hentai manga that has a girl who uses magic (I can't remember if it takes place in high school, but I know it all takes place in a classroom.) (I can't remember if she's a magical girl or just ordinary but has magic.) She first uses her magic to summon a pen, and makes it clear she has to focus to make the magic work. Things turn wild when she hears the guys talking about a guy having a monster penis. Her imagination turns the guy's penis into a sentient monster penis who's perverted thoughts influence her into whatever it says (such as it wanting her cloths to magically disappear).