When the offense lines up offsides when do you flag it? Do you flag and whistle at the snap?

NFHS Ruling
 

<b>NFHS 7-1-6</b><i>
Following the ready-for-play and after the snapper has placed his hand(s) on the ball, <b>encroachment</b> occurs if any other player breaks the plane of the neutral zone.</i>
This is a dead ball foul (encroachment) and the ball would not be permitted to become live.
Also see: <b> 2004 NFHS Case Book 7.1.6 SITUATION A </b>(Play b)

Note to Red Neck Ref:
The term <b>"Offsides"</b><u> is not a used anywhere in the NFHS rule books nor is it used at anytime in NFHS Football.</u>

You say "lined up offsides". I assume (there's that word) that you mean they were lined up in the neutral zone but not in any three-point stances. Like KWH says you can flag it any time after the RFP and the snapper has his hand on the ball. But were the offense to line up in a stance with a hand on the ground then they can't move and can't back out of the neutral zone. You may give them a chance once to get back before they get set but after they are set they can't move and thus can't "unencroach". Flag them as soon as you can so you don't get other things happening like defensive encroachment also.

Are we talking wide receivers here. There you might want to use a little voice first. "Look where you are."

There is not much grey area on the interior lineman you should get that as soon as it happens. I'm speaking of HS or higher ball. With the wide outs you can give a little early and let them know that you will help them. Most of the WRs with ask you "am I on" I always say "this is the line of scrimmage" while tapping my back leg.