A door was smashed open inside the prison at Guy's Marsh, Shaftesbury, Dorset, to get the ladder, which was then propped against the 15ft razor-wire security fence for the escape on Sunday night.
An employee raised the alarm as the youngsters, aged about 17 or 18 and serving sentences of up to a year for offences including taking and driving away, burglary and theft, made off.
A police spokesman said yesterday that a van had been reported stolen shortly afterwards. Three of the fugitives were later recaptured at Salisbury in Wiltshire, he said.
Police are hunting for the remaining seven, six of whom come from south Wales and the other from Bristol.