Firefighters and specialist search and rescue teams were called to the former Duke of Edinburgh pub on Crabtree Street at about 18:00 BST on Monday.
The six 15-year-old boys had been seen "in and around the building" before it collapsed, Lancashire Police said.
No-one is believed to have been seriously hurt.
An investigation into the cause of the collapse is under way.
Six teenagers have been taken to hospital after being rescued from a derelict building that collapsed in Blackburn.