Ref kills live ball on inadvertent horn - sub?

[bucky]

NFHS:

Official bounces the ball to FT shooter A1, who is shooting his last awarded FT. While A1 has the ball, the clock operator buzzes the horn. All 10 players stop in confusion. C blows his whistle and has the FT shooter give the ball to the L. The clock operator indicates that it was an inadvertent horn. As L, holding ball, prepares to administer the FT, B2 rushes to the table to sub. C blows his whistle and beckons B2 into the game.

Legal or illegal substitution?