Kate Thackara, 19, an art student from West London, knows only too well how post-exam results partying can get out of hand. “At a friend's house more than 350 people turned up when only 50 had been invited,” says the 19-year-old who celebrated her A-level grades last year. “Windows were smashed and the whole house was trashed. I was terrified and hid upstairs with my friends. In the end the police turned up and chucked everyone out.

“At another party the kitchen got flooded, food was chucked everywhere and a week after the parents got back they noticed a disgusting smell in the living room, which turned out to be burning sick; someone had thrown up behind the radiator and no one had bothered to clear it up.”

The results for this year's A levels came out on Thursday; GCSEs follow next Thursday. Of course there's anxiety about the grades themselves, but the aftermath of endless partying can be just as worrying, whether teenagers are celebrating or drowning their sorrows. It's particularly true with younger ones who've just finished their GSCEs.

We have all heard of youths who have overdosed on drink or gone psycho on skunk. Lately the anxiety has been upped several notches by headlines such as “Facebook party riot!” and reports of terrifying teen gatherings that have attracted hundreds of gatecrashers after being posted on social networking sites.

The period after results, with emotions running high and a huge release of tension, brings its own particular temptations, says Gael Lindenfield, a psychotherapist and author of Confident Teens (Thorsons). “It's very easy for kids to drink too much or do silly things when they're high because they have got good results or depressed because they have done badly,” she says. “It is a risky business. There are dangers and parents need to take it seriously. The most helpful thing is to prepare with a discussion that parents have thought through beforehand.”

In fact, most teenagers hate parties that get out of control as much as their parents. And it's not just Facebook that is to blame for mega-gatecrashing; word of mouth can lead to chaos too. One solution, if the celebrations are at your house, is not to interfere but be around to step in if things get crazy; a bit like a lifeguard. It helps to have an informal bouncer - someone's older brother, perhaps - on the door to check the guest list. Margaret Lawry, whose son Henry, 18, got his A-level results on Thursday, and whose daughter Heidi, 16, is waiting for her GCSE results next week, roped in one of Henry's friends to sit on the door at Heidi's last party.

“Heidi always seems quite happy that we're around, although she tells us in no uncertain terms to stay in the bedroom,” Lawry says. “She posted the last party on Facebook, and I was worried at first, although I accepted what she said about making sure it was accessible only by her listed friends. The only trouble we've ever had is when a group of teenagers turned up who Heidi didn't know; she was quite shocked. One of the girls had a real attitude, but when my husband asked them to leave they did, luckily.”

The Lawrys also faced the dilemma that plagues all responsible parents: should they provide alcohol for 16-year-olds? Lawry says: “Part of me feels I shouldn't because I'm colluding with under-age drinking, but I feel that whatever you do isn't going to be perfect, and at least there are adults around at our house.

“We provided limited amounts of beer and sparkling wine and told Heidi to tell her friends not to bring alcohol. The trouble is that you always get kids who raid their parents' wines and spirits. One girl got very drunk and I worried about her all night. I couldn't relax until I heard that she was fine the following morning.”

The only way parents can realistically stop teens bringing their own booze is to search their bags. Definitely not a good idea, says Sarah Newton, a teen coach and former policewoman, and the author of Help! My Teenager is an Alien! (Michael Joseph Ltd). “Searching guests' bags shows a lack of respect and trust. I've had to do it myself when I was in the police and it is not to be undertaken lightly.” She believes that parents should keep a sense of perspective, recognise that kids have come through a stressful time and be realistic about teenage partying.

“I think we've gone past the argument of should 16-year-olds drink, because they do. So while I would never condone adults giving alcohol to under-age teenagers, I think you have to recognise that they are drinking so you have to look at ways of making that safe. My own view is that parents would be foolish not to be on the premises during a 16-year-old's party, because if teenagers are drinking at your house, you are responsible for them. You also need to know that their parents are happy with that.”

This is where parents are stuck in a dilemma. If they insist on being around, or come on too strong about what is and isn't allowed, their kids will find a “free” house where there are no adults to cramp their style.

Exam-result celebrations are often impromptu gatherings, held somewhere random, such as a common or a pub near the school, or in my 16-year-old daughter's case, at the Reading Festival (it seemed such a reasonable idea all those months ago, when we were encouraging her to revise). Rather than getting heavy, parents are better advised to make sure that their offspring know how to stop things getting out of hand, and what to do if things go wrong.

“Be positive and don't lecture”

Lindenfield suggests that one parent (with the backing of the other) should agree a quiet time and place for a calm talk, with no distractions. “Start by being positive and establishing that you want to make sure they have fun. Ask them how they think they can stop things getting out of control, and draw them out by suggesting alternatives. Go through your fears and come up with contingency plans: emergency numbers to call if someone gets really drunk, for example. Rather than starting with rules make them feel that they are taking responsibility; that way you're not just saying ‘I trust you', but actually demonstrating it.”

It can be comforting to remember that many teenagers are much more responsible than they are often given credit for, and by 16 they are well aware of the ways that things can go wrong.

When one of 16-year-old Rowan Eggar's friends got drunk at a party, she called her mother, who was away at the time, for advice: “I was quite worried about my friend, who was supposed to be staying with me and Mum told me to send her home in a taxi. We both agreed it was better that she was safe in her own house, with her parents, than at mine, where there weren't any adults around.”

Finally, the worst thing parents can do on results night is to sit at home waiting for the worst to happen. The chances are it won't. Lindenfield recommends distracting yourself by going out and celebrating the results yourselves - just be sure to keep your mobile phone switched on.

If celebrations are at your house

Hide your alcohol stash

Rope in an informal bouncer to sit on the door and check the guest list

Think about how you will turn away uninvited guests

Put out plenty of tempting food; nothing that needs heating up because they won't bother. Don't worry about how nutritious it is - what matters is bulk

Provide bottles of water or soft drinks (nothing dark coloured if you want to save your carpets)

Provide ashtrays

It's the teenagers' responsibility to clear up afterwards - not yours

Make sure they eat before they go out, without making a big deal about it

Above all, your child must feel that he or she can call on you for help, even if they're doing something you don't approve of

If your teenager has drunk too much before, take the number of a sensible friend who you can call for reassurance

Tips from Gael Lindenfield (www.gael-lidenfield.com ) and Sarah Newton (www.theteencoachblog.com )

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