About three years ago, the father of a boy came into the school foyer and asked for me by name. When I arrived, he silently grabbed my tie with both hands and began to choke me.

I tried to get free, but he did a judo back kick that took my legs from under me. He pinned my chest to the floor with his knee and continued to try to strangle me with my tie.

I desperately tried to grapple with him and pull my collar down, but it was difficult because he was quick and strong. I don’t consider myself weak, but the assailant always has the upper hand because they know what they are about to do; the victim can only be reactive.

I shouted and my 62-year-old female secretary appeared. She froze. Two of my male colleagues followed, and the moment the parent saw them he got up and left.

The attack only lasted about a minute but was absolutely terrifying. I was in a severe state of shock afterwards and my colleagues ordered an ambulance for me. I was taken to casualty, examined and released later that night.

I had a mild heart attack a week later. There is some history of heart problems in my family, but the doctor told me that stress from the attack was, without doubt, a contributory factor. I was off work for 11 months.

The father wrote a letter to the school, stating his reasons for the attack. He said that his son came home earlier that day and said I had manhandled him out of the school.

The boy was 15 at the time and I had caught him on camera making a nuisance of himself in areas of the school he shouldn’t have been in, tripping up the junior pupils.

He said that I had dragged him out of the school by his tie, but it was clear from the CCTV images that I had simply asked him to leave the building.

We started police proceedings and eventually the case went to court. The man pleaded guilty to assault, was prosecuted and fined pound;500. I wanted nothing to do with the money and donated it to the local hospice.

I felt a sense of closure, but I wouldn’t go so far as calling it justice. Even today I feel a plethora of emotions: mostly anger and bewilderment. Why did the boy feel he had to lie and why did the father accept his account verbatim and then take the law into his own hands? Why couldn’t he come in and discuss the matter like other parents?

I have never put these questions to the father; nor would I want to. I have never actually had a conversation with the man. The boy was temporarily excluded but was taught in isolation with a one-to-one tutor on his return.

When I eventually returned to school, I couldn’t wear a tie for the first three months, but there comes a time when the principal needs to set a standard of dress. I expect my pupils to dress in a certain way and I now wear a shirt, suit and tie to demonstrate that.

We already had a security door at the school, but our mistake was to push the button and allow this man access without first checking who he was or what he wanted. After the attack, we reviewed our procedures to ensure that our staff and children are as safe as possible.

I have recently moved to another inner-city school, where I am again principal. I wasn’t yet 50 when the attack happened - too young to retire. But I considered quitting on health grounds and filled in the application forms before changing my mind. I enjoy doing what I do too much.

I love seeing young people achieving, sometimes against the odds, and celebrating their success with them. Being assaulted is an unfortunate hazard that comes with the job.

Told to Hannah Frankel. Do you have an experience to share? Email features@tes.co.uk.