As the Home Secretary decides to re-classify cannabis as a class C rather than a class B drug, already the cheers can be heard from teenage users as they look forward to smoking a joint on the way to school, without fear of arrest. Although only a minority would put themselves and their school at risk, the major problem for school staff is in trying to recognise cannabis from any other drug use that is going on in school.

How do we know when drugs are being used around schools? And how best do we respond? The list of physical signs alone is enough to send you dizzy when you include possible indicators in terms of behaviour, emotional states and social interaction. For most drink and drug users, the physical signs of use tend to come with time. So it would be dangerous to assume that all young people with spots, bad breath, mood swings, fluctuating weight and lethargy are on drugs. These things may be incidental.

If we believe the range of surveys that have been published over the past five years, perhaps a third of under 16s will have tried an illegal drug.

By the age of 18, the number rises to about two-thirds.

We should be concerned, but we also need to be understanding about the influences on young people that make drug use a reality for so many - parental use of alcohol, cigarettes and prescribed medication, media advertising of alcohol and the implied messages they contain, peer groups, fashion, music and the curiosity of youth - all this in a society where the range and availability of drugs has never been greater.

It’s the first-time user, the experimenter, who is most at risk; some drugs are volatile substances - lighter gas and aerosols can kill at the outset of a drug career. Approximately 6 to 7 per cent of first time users die from the freezing and consequent asphyxiation caused by propellants. The first-time use of alcohol can often result in disaster from the extreme of alcohol poisoning, which may result in respiratory failure or lead to any number of accidents that occur when inebriated.

It may be that the physical indicators are consistent with the accident that follow drug use. Regular injuries that follow the social cycle may make you concerned. Why, for example, are bruises apparent every Monday.

This may highlight a drug or alcohol problem in the home, and may also be a child protection matter. Behaviour such as violence, abuse, bullying, theft or commercial damage is all heavily linked to drug use. So too are more worrying signs such as depression, mental health and even suicide.

We need to develop more awareness of the effects of drugs. There are drugs that depress the central nervous system, others that stimulate it, often resulting in lethargy, paranoia and depression with continued use. But it’s difficult to be sure that drugs are the key to mood swings or physical health. For instance, cannabis may well induce a laid-back euphoria and outbreaks of hilarity, as might alcohol or even small quantities of heroin.

Reddened eyes could be caused by drugs or hay fever, conjunctivitis or even crying. “Pin-prick” pupils may indicate heroin use or sensitivity to light.

Dilated pupils could be caused by cannabis, amphetamines or ecstasy, but it could be caused by a number of other medical problems, including head injuries.

Despite the scary surveys, it’s important not to assume that drug misuse is responsible for a pupil looking rough or behaving strangely. Knowing the paraphernalia is useful:l Empty plastic (cider) bottles with the bottoms cut off for use with a bucket of water, or with a plastic bag stuck to it to make a “lung”; * Ripped cigarettes that have been emptied and re-rolled to make a joint; * Foil, flat with sticky black patches may be the left-overs from heroin smoking. Needles and syringes are far more obvious; * Razor blades for cutting powders, burnt spoons for “cooking”, ready for injecting.

You may even see the drugs themselves from marijuana to cannabis resin, any powder or tablet to a piece of paper with a bright picture on it. You need an idea of what you are looking at.

To spot drug misuse, it helps if you can assess the dynamics between groups of young people and their family relationships. And be aware of mental health problems.

Any changes may offer clues that will help to put the puzzle together. When you do challenge behaviour, use the policies that are probably already in place in school. Users may respond with truancy, abusive language or violence, but the consequences are for them to face up to. It is not easy to deal with but neither is it impossible. When schools start developing links with local services and getting armed with comprehensive drugs and behaviour policies it is possible to make inroads. The more knowledge schools have at their disposal, the more confident they will be to address these problems. The more we know, the less we will worry that mood swings and arguments are normal signs of adolescent development, and that spots on faces are caused by acne.

John Taylor is a schools drugs adviser in Nottinghamshire