Mimumu to Shirara: Dragon no Chinchin wo Mi ni Ikou

Shirara and Mimumu are first place and second place respectively in their magic school. In order to earn good grades in their transformation class, they want to reproduce "every nook and cranny" of the transformation subject. Dragon, unicorn, incubus, kraken, demon, slime... just how do their penises look like? And so, these two girls start their ecological research in this other world.

(Source: Shinchosha, translated)