drop

Snape gave Harry a swift, piercing look. Harry looked at the floor. He wished Ron would put his wand down.
Then a small voice came out of the shadows.
"Please, Professor McGonagall -- they were looking for me."
"Miss Granger!"
Hermione had managed to get to her feet at last.
“I went looking for the troll because I - I thought I could deal with it on my own - you know, because I've read all about them."
Ron dropped his wand. Hermione Granger, telling a downright lie to a teacher?

If the dictionary say like this on “drop”:
to let or cause to fall.
to cause or allow to sink to a lower position.
http://dictionary.reference.com/browse/drop?s=t

Don’t we know whether Ron’s wand went down off his hand or not?