There are no specific UIL rules that address the topic of using flash cameras from the sidelines of a football contest.
Any and all photographers taking pictures of anyone other than their own children must have permission from both schools involved in the contest and a model release to use the pictures for anything other than personal viewing. Video falls under the same restrictions. Schools may grant media permission to take pictures for publication or broadcast purposes.
Officials on field have the ability to move photographers away from the playing surface if determined they are interferring with the flow of the game or for safety purposes.