I'm not sure if you're asking for the name but, are you perhaps talking about Uzumaki? By Juni Ito?
https://www.mangago.zone/read-manga/uzumaki/
Well technically it's a snail but it's not boy or school either so no
Theres no boy or school? In the post you stated that the story takes place at a school and the boy was stomping on snail eggs. Unless I'm reading wrong?
I take my words back, I sound like a hypocrite.
I think it's multiple horror stories but this one chapter keep making me a nightmare about it until now, the story place in school with one boy in raining days he was stomping snail egg on the drain like for fun then he started laying snail egg eventually become mama snail himself and goes to the drainage while it's raining.