If you watch the video with the school's AD, he states that the team did a tribute similar (but smaller) to this after a PREVIOUS scoring play and the officials let it go but told them not to do it again. They did it again and were flagged.
In addition, you can see there is much more going on during that celebration than just a kid pointing to the sky. One kid jumps on other players a few seconds in, and another is pointing to the crowd behind the endzone.