Perhaps it was dark and the schoolmaster couldn't see what was in front of him.
Apparently, a body was lying at his feet, but at first he could not make it out (see it, or identify what or who it was.)
"Lie the body at one's feet" does not exactly apply here. "To lie" is intransitive. "The dog lies on the rug."
"To lay" is transitive. "The president lays a wreath at the tomb of the unknown soldier."
The dog is lying at my feet. The dog lay at my feet. (past tense)
Lay the book on the table. He laid the book on the table. (past tense)