Nanatsuiro Dropsななついろ★ドロップス; ななついろ☆ドロップス; Seven-Colored Drops

Based on the Eroge by UNiSONSHIFT. We've got our main character, Tsuwabaki, a normal student, but one day he bumps into a weird guy with "not cosplay ears" who gives him a drink. It transforms Tsuwabaki into a chibi lamb! The teacher explains everything, and the teacher tells him the only way he can get back to normal is to find a chosen girl and have her catch the 7 Stardrops. The only thing is, is he can't tell his true identity.