Too Late For 11th Man To Enter Field?

[Blue37]

I was watching my son's high school team last night (fed rules) and something interesting happened. We were on defense and the other team was punting. We only had ten players on the field and the 11th started to run on, but the linesman prevented him from doing so by sticking his arm out and blocking his path. There was no contact between the player and the official. The linesman then said something over his shoulder to the coach. Afer the game, I asked about the situation and was told the linesman said defensive players were not allowed to enter the field after the offense broke the huddle.

Is that the rule? As long as it is an honest mistake and not an attempt to gain an advantage, is it ever too late for the 11th player to enter the field? What about if the ball has been snapped? Is NCAA different?

Thanks in advance.

PS - We won the game so this is not sour grapes. Just an honest desire to learn.