More info coming out:

Denver Public Schools officials said Weld Central High School fans brought a Confederate flag to their home game at Manual High School...Denver7 spoke with Weld Central High School’s assistant principal and Weld County Public School’s athletic director Scott Richardson about the alleged incident.

“I didn't see any Confederate flags, and I asked the coaches and they said they didn't see any flags,” Richardson (Weld County Public School’s athletic director) said. “If we did see it, we would have asked our fans to put it away because we banned the flags.” ... “... our players need to know it’s not acceptable and there’s no place at Weld Central for these things,” Richardson said...we definitely sit our players down and address them,” Richardson said...
http://www.thedenverchannel.com/news/local-news/dps-responds-to-alleged-confederate-flag-and-racial-slurs-at-football-game