But they wouldn't do that, because the penalty would be an awarded touchdown. Isn't that the rule for when a player comes off the bench to stop an apparent touchdown run?

Do you think #50 entered the field with the intent to substitute? Or Celebrate? That's the question. If he went onto the field to replace a player, then yes, invoke 3.7.6. BUT, if he was on the field for no other reason than to celebrate what he thought was a game-ending play, then you have to invoke 9.8.1 -
"ART. 1 . . . No coach, substitute, trainer or other team attendant shall act in an unsportsmanlike manner once the officials assume authority for the contest. Examples are, but not limited to: i. Being on the field except as a substitute or replaced player."

In my opinion, neither #50 or any other B player went onto the field to take the place of a player who was already in the game, therefore, the only explanation is that they were nonplayers who were on the field and not substitutes or replaced players.

Boy, me or you one needs a new rule book.
First of all, there is no rule that gives an official authority to blow a play dead, except for a RUNNER whose helmet has come off.
Secondly, if an official does blow it dead, it's an inadvertent whistle (vulgar language where i come from :))
Third, you are correct in your interpretation of a player coming off the bench to stop an apparent touchdown run. If a sub came out and tackled the ball carrier when it was obvious he would have scored, then the Referee can invoke the unfair act rule and award a touchdown.

But the proper call for nonplayers on the field with their hats off during a touchdown scoring play is to let the runner score, then give A the option to accept the USC foul on either the try or the kickoff.
All this is black and white in the 2009 copy of the NFHS rule book.

Anyone know what caused the USC?

We had a very similar situation happen to us this year when a team thought the game was over and came running on the field. They realized the ball was still live so they ran back off and never affected the play. Team A didn't score so the game ended but their coaches were screaming at us for not flagging them. We talked about it in the locker room and knew that even if we flagged it, it would not have extended the period so the game was over either way. What we struggled with was what foul would actually apply. We ran through everything that has been mentioned here (illegal substitution, illegal participation, USC, even SLW). We checked with several other officials as well and never really got a definitive answer.

Actually, it did make a difference. If you had thrown the flag for illegal substitution or illegal participation, since both of those are live ball fouls, A would have had the opportunity to run an untimed down. If you got them for nonplayers being on the field, that's treated as a dead ball unsportsmanlike, therefore no untimed down.
3.2. ART. 3 . . . A period must be extended by an untimed down if during the last timed down of the period, one of the following occured:
a. There was a foul by either team and the penalty is accepted, except for
those fouls listed in 3-3-4b.

ART. 4 . . . A period shall not be extended by an untimed down if during the
last timed down of the period, one of the following occurs:
a. When the defense fouls during a successful try/field goal and the offended
team accepts the results of the play with enforcement of the penalty from
the succeeding spot.
b. There was a foul by either team and the penalty is accepted for:
1. unsportsmanlike fouls,
2. non-player fouls,
3. fouls that specify a loss of down, or
4. fouls that are enforced on the subsequent kickoff as in Rule 8-2-2.
NOTE: The score is cancelled in the event of an accepted foul that specifies a loss of down.

Actually the illegal substitution foul we looked at was 3-7-6 "During a down, a replaced player or substitute who enters the field, but does not participate, constitutes illegal substitution." In the penalty section for this rule it indicates this is considered a non-player foul. We couldn't find an IP rule that fit since the players did not participate. So that left us with USC or IS non-player both of which fall under the rule you state above to NOT extend the period. What I would like to know is an official word on what rule applies when this happens.

So if the marching band comes onto the field thinking the ball is dead and one of them is likely to have his trombone slid up his wazoo, you let play continue?

Yes, see Stanford/Cal. :D

You are absolutely correct. Please accept my apology.:o

The only option that would have extended the period would have been illegal participation.

Would you consider this USC, IS, SLW, or nothing considering it was the last play of the game and enforcement would not extend the game? Assuming it's not the last play of the game, does that change your ruling?

I agree with the sequence. I do believe that defense committed a live ball foul because the number of subsititutes and proximity to holder influenced the play. So you wind up with a double foul which may have thwarted the USC penalty that followed.

Honestly, if they just overreacted to the play and were on the field because of their emotions, I wouldn't throw a flag. With the ball and the runner headed toward the goal line beyond the team box area, I would be concentrating more on what was going on down there instead of looking over my shoulder worried about nonplayers on the field. JMO.

Also, whether it was last play or not, I hope my cal/non call would not be affected by clock status, but I really don't know. I would have to be there.
Since you guys did nothing, I'm going to defer to your judgment because you were there. More than likely you did the right thing.:)