Fortunately, NFHS soccer rules for substitutions are similar to those of basketball. Soccer subs also don't report whom they're replacing; they just show up at the table (if there is one), like basketball, and aren't players until they're beckoned in. (The big difference is, when the clock is running, a soccer sub has to be at the table area when the ball goes out of bounds.)

In either sport, the way I've read it is, once they're beckoned in, someone has to go out. Or, as PG mentioned, someone else could come off the bench to replace the new player getting "pulled back." (Though, I believe you could only do that in soccer with a stopped clock.)

Same goes for NCAA soccer. FIFA Laws, though, require that the player leave the field before the substitute may enter. This requirement is often relaxed at lower levels. When we do relax that requirement, we consider the substitute a player when he enters the field with the referee's permission. The player going out, at that point, is simply a substituted player who is on the field with the referee's permission. Play won't be restarted until that substitute player has left the field.

Also, since in NFHS and NCAA substitutes become players when beckoned by the referee, the player they're going in for has to leave. NCAA provides an exception to this -- the substitute may be withdrawn after being beckoned and the player they're supposed to be going in for may remain a player, but the substitute is charged with an entry. NFHS doesn't have this exception, probably because there are no limits on how many times a player may enter the game.