I'm willing to bet that the bunny costumes were bought by the previous students from a shady alley shop, and were cursed (blessed?) so that any person who wears them has their desires amplified by a large amount. Some orgy probably happened between the supervising teacher and the students that made them not use them. The girl here seemed to go from extremely timid and embarrassed to horny and willing in record time. Just a theory, though.
"A bunny girl just confessed to me with her hand on my dick"?
Amazing. Truly only something that could only happen to a Hentai Protagonist.
Great ending though. Well hey, why stop at just a bunny outfit? I'm sure there are a ton of great outfits she'd be willing to wear for ya...