Both my son and I sometimes forget to reset our alarms for the weekends so that could be a possibility... and yes, those dang things blare until you physically turn them off!
I also read where she had just made the cheerleading squad so maybe they had an early morning practice before it got too hot outside. The football team does that here - they even had a midnight practice a few weeks ago; the cheerleaders where there too.
Another thought is that about week before school began my son started getting his circadian rhythm back in sync with regular school hours so it would be easier to adjust to getting up early. Sometimes I forget he is a teenage boy because he often acts older than his own mother.