Holding in the endzone

[IAUMP]

For the past week crew chief and I have been debating the penalty enforcement of a play we saw in an AF2 game. We are discussing the play as we would apply the rules in a Fed High School game. Here is the play.

3rd down and 15 from A's 1 yard line. As A1 drops back to pass A50 holds B99 in the endzone before the pass is thrown. Please respond as to your placement if B were to accept the penalty.