My daughter is due to start high school in September and I have just found out that I know one of the teachers at the school.

This teacher had an affair with one of his students when she was 16. She is a friend of mine and this relationship has had a huge – negative – effect on her life. I am very angry at the teacher who did this to her. I have thought of speaking out but I couldn’t do that to my friend. At the same time, I do not want this man anywhere near my daughter. I don’t think he’s a paedophile, I think he’s an arrogant idiot who made one very serious error in judgment a long time ago. (But the thought plays out, what if I’m wrong? What if he’s done this to other students?)

I’ve considered writing to him and telling him what I know, and that he had better keep away, but I don’t want to start something I can’t finish. I can’t stand the thought of seeing him again, at open evenings etc. I don’t know if he’d recognise me now but I would certainly know him and I don’t trust myself to be civil. J, via email

You don’t say when this was other than “a long time ago”. Perhaps if I talk you through what you can do about it, it will help you decide.

I consulted Richard Atkinson, chair of the Law Society’s criminal law committee and a partner at Robin Murray & Co. I also talked to the Department for Education (DfE).

If this occurred before 2001 then no law would have been broken provided the pupil was over 16 and gave consent. However, the school could still have sacked the teacher for gross misconduct, and it could still be reported if this is what you decide to do. Before 2001, the secretary of state could also bar a teacher on list 99 for misconduct. List 99 no longer exists but it was a list of people who were banned from working with children. It was replaced by the children’s barred list in 2009 and all schools must check someone is not on it before employing them.

“That function,” the DfE tells me, “is now with the National College for Teaching & Leadership (nationalcollege.org.uk) for professional conduct and with the Disclosure and Barring Service (DBS) for risk of harm to children. The Disclosure and Barring Service bans a person from working closely and regularly with children, by placing them on the children’s barred list – if it decides that the person poses a risk of harm to children and that it is appropriate to do so.”

Since 2001, a teacher having a relationship with a pupil, even if that pupil is over 16 and gives consent, is a criminal offence under the 2003 Sexual Offences Act, under the abuse of trust section (this part actually came into force in 2001).

A current employer could sack a teacher now if it “found out a failure to disclose, when applying for the current job, relevant past misconduct in a similar job,” the DfE told me.

So, what do you do? You could pass this information on to your local authority’s social services and they would then decide whether to refer the teacher to the DBS for possible barring. It’s very likely that you and your friend will be asked to provide a statement. If the DBS had other allegations on file about this teacher, then this could help to establish a pattern of behaviour even if any one incident is not enough to bar him.

Things to consider are whether your friend would want to make a statement? You say that you couldn’t do this to your friend, but I’m unsure what you want to happen. Your letter appears as you sent it, in its entirety, so I only had what you told me to go on.

In your situation, I would make sure I had all the facts straight. I would speak to my friend and ask her what she wanted to do about it. You will struggle to make any progress if she won’t make a statement, so her co-operation is pretty key.

Of course, the teacher may have declared this affair and behaved impeccably since. I’m wondering also, why now? If you thought this teacher was a threat to other students why didn’t you do something about it before now, or is it just because he’s now possibly a threat to your daughter? I can see that your child starting at a school where this man works has focused your attention on this matter, and possibly brought up your friend’s distress, but I hope that if you thought him a serious threat you’d have done something before now. If your daughter were not going to this school, would you consider doing anything? I hope this information helps you come to a decision.

First published in the Guardian Family section, 17 May 2013.