Three tired but happy 14and 15-year-olds piled into Angela's car after a junior disco in a local school, on their way home to a sleepover in her house. The next day, her daughter told her quietly not to mention the disco if she ran into X's mother - because X, she revealed, wasn't allowed to go to discos until she was 16.

Another dilemma: when Dermot's son turned 17, his social life picked up. Dermot allowed him to go to a local pub where all his friends hung out at weekends, and accepted that his son had a few beers on a night out - though he would have preferred him to wait until he was 18 to start drinking. It didn't dawn on Dermot for a while that the reason one of his son's friends had become a regular overnight visitor on weekends was because the lad's parents would have been furious if they had known he was drinking.

Should you tell? If you were Angela, for example, would you risk losing your own daughter's trust by ratting on her friend - knowing that she might never trust you with information again?

Would the fact that you think the friend's parents are over-strict affect your decision? Would it depend on how well you knew the other parents? Or on whether you thought the child's behaviour was leading into danger?

No parent likes to collude in deceiving another parent, and most wouldn't agree in advance to do so. But many parents will stumble across information about their teenagers' friends that may turn out to be guilty knowledge.

Welcome to the parents' moral maze. Talking to several parents of teenagers, I found no simple consensus on this issue. Paula, a mother who had tangled with various teen-parenting issues as the facilitator of a parent support group, says, "If I were the parent being deceived - about the disco, say, or the drinking - I'd want to know. "Ninety per cent of me would want to talk to the other parent, and I think that parents would and should be loyal to other parents. But then, if I didn't know the other parents, I'd probably chicken out of it. I think I would talk to the child concerned, though, and warn them that next time `I'll check with your mother'."

These are issues likely to arise as your children reach the ages of 14, 15, and 16, as they gain more independence - and more opportunity to do things they know you'd object to.

Teenage deception of parents is fairly common; it's brave parents who assert that their teenager has never, ever lied to them. So should you act if you come across a lie? Marie Murray, senior clinical psychologist in St Vincent's Psychiatric Hospital, Dublin, says "there are no rules for this, apart from some essential guidelines around risk. If a child is engaged in an activity that's dangerous or detrimental to themselves or others - drugs, criminal activity - or is suicidal or depressed, then it's up to adults to guard and protect them."

But what about grey areas? Well, you could try to intervene to stop your own child colluding in a deception about an activity you think is safe (going to the junior disco is a good example) by seizing the initiative. "You could ring the other parent and tell them you've checked out the disco, you're letting your own child go, and you'd like to bring and collect her friend too," Murray suggests. This at least gives the child the chance to go to the disco without deceiving his/her parents.

Colluding to deceive another parent in advance is definitely not appropriate behaviour, Murray says. However, some parents are unreasonably strict and excessively punitive; they leave their children little alternative but to deceive them.

Another adult is likely to weigh this in the balance before deciding to "tell" on a teenager.

Context is also important. "Say you stumbled across information out of context - for example, you saw a good, normally well-behaved young person smoking once. You'd have to make a judgment about the reception you'd get if you told the parents and whether it would be useful to the young person."

The point, Murray says, is that these are far from being open-and-shut matters. Parents will have to dig deep into their own ethical selves to come to a decision.

As for the trust between you and your own teenager, it might be worth discussing these issues with them before they arise. In particular, make it clear that there are confidences, especially those involving another child's safety, that you cannot keep.

"If a friend of your child is depressed, or perhaps suicidal, this is always an instance in which action of some sort is required," Murray says. It is wrong, too, to turn a blind eye to teen drinking, she says.

Sean Mitchell, president of the National Parents Council (Post Primary) suggests it is possible to tackle a problem like this with a degree of subtlety. If the issue was a teenager drinking, "I wouldn't confront a parent and say `Did you know your lad was drinking?' After all, it's everyone's responsibility to be aware of what's going on in his or her own family, and you risk losing an adult friend if you expose inadequacy as a parent. But I would raise the issue, perhaps by saying something like `My own lad has a tendency to drink too much'.

"It's a question of being tactful and hoping that they'll pick up your hints and do something about it."

Mitchell, whose own youngest child is now 18, would be loathe to go beyond such hints. "My own reaction is that I'd be slow to rat on a teenager, because of the risk of losing young people's trust. But there were situations where I'd have issued warning directly to a child, as well as getting an older teenager to keep an eye on the child for me."