Katatsumuri Zensen

High-school student Wakamori Asako has always been watching Sakura from the athletic department after she comes home from school. One day, however, Asako comes across a snail... who can talk! The snail is actually her schoolmate Kajiwara Atsushi, an omnipotent athlete with excellent grades whom the girls treat like a celebrity. However, stress causes Atsushi to turn into a snail. Asako becomes a "snail helper" to aid Atsushi while he is in this form, but she might cause Sakura to misunderstand...

(from mangaupdates)