Conor Baker and Danny Keogh, both 16, were riding the Mega Bounce Frog at Abbey Park in Leicester in July 2012 when their carriage tipped over and flung them out.
Danny suffered a collapsed lung, broken arm and deep wounds to his side. He needed 88 staples after extensive surgery, as well as stitches in his tongue and chin.
Conor suffered a broken nose, broken teeth, lacerations to his face and a significant blow to the head.
The ride's owner, William Norman Roberts, from Atherton, Greater Manchester, appeared in court after the Health and Safety Executive (HSE) launched an investigation and found that a clip and pin securing the ride's car to its operating arm had fallen out.
They also found the ride had been badly maintained.
At Leicester Magistrates Court , Roberts, 36, admitted a health and safety offence and was fined £6,500 and ordered to pay costs of £5,000.
After the hearing, HSE inspector Neil Ward said: "The unsafe condition of the ride resulted in very serious injuries to two young people who had gone out to celebrate finishing their GCSE exams.
"The outcome could have been much worse. It was a horrifying and painful experience for the two boys and extremely distressing for their families, friends and members of the public who witnessed the incident."