I've always wondered if Mark, just before his final broadcast, told his parents the whole truth. It would seem that way because Mark tells his girlfriend his mom let him use the Jeep ("She kinda loaned it to me"). Also, his Dad was at the gathering in the school's athletic field, but there is no shot of him acting surprised or horrified when Mark pulls in to where the crowd is and gets arrested. So the question is: did Mark fess up to his parents? Or is it irrelevant/left for us to wonder?
[Well, with no actual scene where he confesses, it's left up to us to wonder. Personally, I find it unlikely that he'd actually admit the whole thing to his parents, but they're not stupid and already had their suspicions, so the lack of any great surprise on his father's part isn't unreasonable. You also have to remember that Mark's voice changer had already broken before they drove down to the crowd - his father would have easily recognised his voice before his actual arrival, giving him a certain amount of time to get through the initial shock.]
Answered by Tailkinker