Shot Clock Question

[Zebra1]

I just got my season started last week already! Of course we are going to have problems working the kinks out. We had a play and me being the rookie just put my 2 cents in and went on, but I don't think we got it right. The play:

Shot clock winding down, ball goes out of bounds, whistle just before the horn about as close as can get. We get together, I'm thinking probably put 1 tick on the shot. Nope the other 2 both disagreed and said violation. I know it wasn't a full second, but don't we have to give the ball back with no violation. Some veteran input please.