Buried 'at' a cemetery.

Buried in the school cemetery. He is buried in the cemetery there. The boys were buried in the cemetery. She buried the box in the back garden. Buried her face in the pillow. Buried the secret deep within himself. Buried myself in my studies/work/book.

A person's body may be interred (buried), so we naturally use "in". Surrounded by earth. "Buried at ..." usually refers to the burial ceremony. We do say "buried at sea". We can only say "buried on (place)" in the sense of "on top of something". So, "buried on a hill/mountain" or "buried on an island" make sense, but not "buried on a cemetery". I'd think the corpse was not buried, but exposed.