The NASUWT regional secretary for Stockton-on-Tees collapsed with chest pains in a service station on the way home from a union annual conference and was taken to hospital where she was diagnosed as suffering from a stress-induced ulcer.
"Over six years my line manager gradually and systematically wore me down.
Why did I put up with it? I thought I was a strong person. I was used to dealing with other people's problems."
She said her line manager at Stockton-on-Tees youth service, who was eventually suspended and then retired early, went through her drawers, told lies about her and used appraisal against her.
Ms Percival, who is now co-ordinator for the anxious pupils and pregnant schoolgirls unit, a pupil referral unit in Stockton-on-Tees, revealed details of the bullying she suffered during a conference debate.
The union heard that management bullying was rife in schools. A questionnaire in Devon which drew 700 responses exposed 989 instances of bullying ranging from public humiliation to teachers being ignored.
Bullied teachers have had breakdowns, lost their jobs, attempted and committed suicide.
Peter Johnson, a delegate from Bromley, south east London, said: "If any occupation was safe from bullying you would think that it would be education but sadly bullying is widespread. It is difficult to deal with as it thrives on secrecy."
Research for the NASUWT has identified four types of bully:
* the situational bully, who passes threats on to other staff
* the role-playing bully, who thinks it is expected of them
* the punishing bully, who uses punishments rather than rewards, and
* the pathological bully who does it for pleasure.
Delegates backed a high-profile campaign to highlight management bullying and expose how some governing bodies, councils, corporations, learning and skills councils and church authorities try to cover it up.
Alison Sloan, from the Peak District, said teachers wanted leadership not tyranny in schools. "Stop the tyrants," she urged the conference.