Why wasn’t Bryon able to stay with Cathy at the dance?

Bryon's on the rebound after being unceremoniously dumped by Angela. Still somewhat unsure of himself around girls, he takes Cathy to a dance one Saturday night. Once there, he takes the opportunity to get back at Angela, gleefully reminding her of how Ponyboy rejected her after she broke it off with him. Despite this tense moment, the atmosphere at the dance is generally upbeat, and everyone seems to be enjoying themselves. For his part, Bryon is gaining in confidence around Cathy as he realizes just how much he really likes her.
However, this being an S.E. Hinton story, trouble is never far away. The joyful mood of the dance is suddenly disrupted by a blood-curdling scream. It's Bryon's friend, Mark. He's been knocked unconscious with a bottle, hit over the head by some thug that Angela used to get back at Ponyboy. But Mark got in the way, and now he's lying on the ground unconscious in a pool of blood. There's no more time for dancing; the party's over. So Bryon springs into action, and whisks Mark away to the nearest hospital, where he receives stitches to his head wound.