My horse bucks when in the school?

My horse is a 17 year old ex race horse that was mistreated. I first knew him when i started to learn to ride at a local riding school. I learnt most things on him. The owner moved yards and i didnt see him for around 5 years untill i bought him off his old owner. I knew he could buck. However, he never bucked like he does now. When ever im in the school he chucks in a massive buck and always gets me off. He done this once and i broke my collar bone causing me to have a metal plate put in. I have had his back checked, teeth etc. I thought maybe it was down to when he was in the riding school and that he do not like being schooled. I was advised just to walk him around doing lots of circles changes of reins etc. I would really appreciate any advice and help anyone could give me. Thanks.