The four-piece - one of three acts competing in the final of the ITV contest this weekend - were putting on a free show for more than 1,000 fans at Fairfield Halls in Croydon when the crowd surge occured.
"I'd been queuing since 1pm but when the school kids turned up it became so busy with them all pushing us towards the front," one teenager told The Croydon Guardian. "There was a pregnant woman in the crowd. It was horrible. I couldn't move. I couldn't get out."
Four ambulances and three fast response cars arrived on the scene as the large crowd grew. Six people were treated at the scene, including one with breathing difficulties and one with abdominal pain.
"All are believed to be minor injuries," said a police spokeswoman. "The most serious injury is thought to be a suspected fractured wrist."