I liked it. Sad. I'll admit it's not one of your best pieces, at least in my opinion, but still quite good. I think the fact that they're so young yet still have sex is a bit... offputting, even as sweetly as you put it.

The time the story takes place is a little confusing. It starts off as Autumn, but then at one point you mention the "approaching September breezes" as if that month hasn't even arrived yet, and that they spend "all of the dying summer" together. I know I don't consider it Autumn until late September or early October, so maybe you could do something about that to make things clearer.

"I cry at night and they tease me for being homesick, not knowing that I'm sobbing because Cassie will be too, wondering why I left her."
This line is a little vague. Is he sobbing because Cassie is sobbing, or because she's homesick? It sounds more like she's homesick, than crying too.

The narrator's simple thoughts and views of the world around him accentuate his young age and is very appropriate and well done.

While a somber read through most of it, with an even more somber ending, the small snippets of fancy, like their building little castles for the ladybirds add a gentle twist to it. A very nice piece.