Niboshi-kun no Hentai

Nekota Kuniboshi (aka Niboshi) is known for his perversion. All the girls at his school despise him, except for Yumeno Shizuku. Shizuku is nice, smart, and the prettiest girl. The fact that she talks to him confuses Niboshi to no end, and he has no idea how to deal with it. While Shizuku is away from her desk, Niboshi finds a half empty water bottle. He drinks out of it and proceeds to stalk Shizuku while she practices her gymnastics. As he watches her practice, he wishes that he could turn into the suit that she is wearing. To his surprise, his wish comes true! Now, he finds out that the liquid he drank from the water bottle was in fact, a chemical created by Shizuku's father that allows him to turn into any object he wishes. An odd experimental relationship begins between Niboshi and Shizuku.

(Source: MangaHelpers)