Had this situation in an 8th grade game recently:

A's ball 2nd and goal at B's 6 yard line. B notices that they have only 10 defensive players. So as A is coming to the line and after the ready for play, B11 runs onto the field. He doesn't make it to his side of the line before the snap.

B2 runs 6 yards for a touchdown.

My partner flags it, calls offside (actually encroachment, there is no "offside" anymore, right?) and A declines it and takes the touchdown.

I ask my partner, "Isn't that a dead-ball foul?" He says, "No."

Now, if it's encroachment, doesn't it have to be a dead-ball foul (no play, no touchdown)? And if it's a substitution infraction, isn't it also a dead-ball foul?

A won by 34 points, so it was not the biggest deal in the world. But if it happens again, I want to make sure I get the call correct. What do y'all think?

it is illegal participation and is enforced from the spot that you noticed the infraction (where the flag is)

this could be a sticky situation.

Canadian Ruling
 

A will decline B's Illegal Substitution foul and accept the touchdown.

michaelpr, this is not an IP foul, it is an IS foul. It is IP if he enters after the snap, but if he enters before the snap and is not on "his" side of the NZ, it is IS. See 3-7-5 in rule book, and 3.7.6 in case book.

Agree with MJT. It is also a live ball foul, so the score stands if A declines.

Live ball foul for illegal substitution (one of the two instances that you can have live ball illegal substitution if I'm not mistaken).

A good way to remember that this is not encroachment is that he's technically not a "player" until he is on his team's side of the neutral zone. Thus, it's a live ball illegal substitution.

How did B2 run only 6 yards for a TD? Seems to me that B2 woulda had to have run 94 after picking up a fumble or making an interception or something.

Sorry. A2 ran for the touchdown. It was late, I was tired.

So live ball foul was correct. The "offside" call was incorrect? Should have been illegal substitution?

We also had a 15-yard face mask at the 20 going in and my partner wanted to mark it off down to the five yard line, so I just wanted to check to make sure.

Half the distance on the face mask penalty, ball on 10 yard line.

Agree with MJT, IS, live ball, TD is good after penalty is refused.

But didn't B foul on a scoring play by A where there was no change of possession? The score for A stands and the penalty for IS is enforced on the try.

If he tries to participate after entering it becomes illegal participation, right?

We have a winner!

Yes - he's a substitute until he gets to his side of the neutral zone, and substitutes may not participate.

if this is a live ball penalty, why would the enforcement be from the previous spot as stated in 3.7.6 of the case book and not from the end of the run? that is really not penalizing the defense as much as it should be.

It's a foul simultaneous with the snap; all such fouls are penalized from the previous spot.