They are the most secretive of illnesses that pupils go to great lengths to keep concealed from their teachers, their parents, their family and their friends.
Eating disorders are estimated to affect more than 1.1 million people in the UK, with the vast majority developing a condition between the ages of 13 and 25. They have the highest mortality rate of any psychiatric illness.
One in five of those seriously affected die, and rates of suicide are 200 per cent higher than the general population.
Teachers are often the first people to suspect that a pupil may be suffering from anorexia, bulimia, binge eating, compulsive eating, or one of the multitude of variants of these conditions.
Of the thousands of calls received by an eating disorders helpline, one-third are from teachers or other professionals involved with young people, with the remaining two-thirds coming from the sufferers themselves, their friends or carers.
The likelihood is that eating disorders are an issue which will arise within every secondary school. The earlier they are identified, the greater the chances are that the young person will make a complete recovery.
Julia Hathaway is head of training at the Eating Disorders Association, the only national charity providing information and support to people affected.
She said teachers should become concerned when a pupil shows changes in behaviour or changes in physical appearance, or if the young person is failing to thrive.
"An eating disorder is a mental illness and it should be taken seriously,"
she said. "Teachers are in a unique position to support a sufferer because they build up a trusting long-term relationship with pupils and do recognise the changes in behaviour or appearance.
"It may be that a child develops irrational mood swings, or devious and deceptive behaviour. It might involve stealing food or trying to get rid of food. They might believe they are ugly or fat or underweight."
A sense of low self-worth is a constant feature among sufferers. Exam stress, family break-ups, or bullying and abuse, can all act as triggers.
About 10 per cent of sufferers are male. The majority of the men reported that their eating disorders had started in their school years when they were overweight and bullied or called names.
Louisa Muspratt, the EDA's professional support co-ordinator, said both anorexia and bulimia have specific physical symptoms. "While anorexia usually results in severe weight loss through the restriction of food, a student may also exhibit other tell-tale signs including fainting and dizziness. They may start exercising excessively, skipping lunch or becoming ritualistic around their treatment of food.
"It is often the case that someone with anorexia is considered to be a high-achiever or perfectionist and they will almost certainly have a distorted body image, believing themselves to be overweight."
She said young people with bulimia can be much harder to distinguish, as sufferers generally will maintain an average body weight. Bulimia occurs eight to 10 times more frequently than anorexia, but the lack of external indicators increases the risk of the condition remaining hidden for longer.
"It involves consuming vast quantities of food. The sufferer will then look for ways to rid themselves of this through inducing vomiting, abusing laxatives, or exercising," she added. "A cycle of bingeing and purging perpetuates the sufferer's belief that they are out of control.
"In turn, this leads to them becoming increasingly depressed and disgusted by their own behaviour. If you suspect a student may have bulimia you may notice that they often suffer from a sore throat or tooth decay and are likely to be dehydrated."
The EDA has produced a five-step guide for teachers who suspect a pupil in their school has an eating disorder, the first being to contact the organisation which can provide you with essential information and support for both staff and students. It also has access to a database of professional support services across the UK.
The second is "voice your concerns". Approach the issue sensitively and encourage the sufferer to tell their parents or carer and support them in doing so. If the student does not admit there is a problem, consider seeking professional support. It is dangerous to let an eating disorder go unchallenged.
The third is "share responsibility". Gaining the trust of the student is essential. If you feel a student is at risk, let them know you may need to inform another member of staff about your concerns.
While it is important that student confidentiality is respected, you should not carry the burden of supporting a student alone.
The fourth is "create a policy" for eating disorders - few schools have one. The EDA will help with this, and it should be circulated to all staff, parents and students. It is vital that you do not wait for a problem to arise before a policy is established.
Finally, "raise awareness within the school". The Eating Disorders Association has produced an education resource pack and can provide posters and leaflets for the school library and display boards.
Ms Muspratt added: "The severity of eating disorders and the distress caused to those affected is greatly underestimated. Eating disorders are not about food, they are about feelings, and food is a coping mechanism by which sufferers deal with their internal distress."
EDA Youth helpline (18 years and under): 0845 634 7650; www.edauk.com
The danger signs:
* Substantial weight loss
* Constipation and abdominal pains
* Dizzy spells and feeling faint
* Bloated stomach, puffy face and ankles
* Downy hair on the body
* Occasionally loss of hair when recovering
* Poor circulation and feeling cold
* Dry, rough, or discoloured skin
* Wearing big baggy clothes
* Restlessness and hyperactivity.
* Bingeing and vomiting
* Disappearing to the toilet after meals to vomit
* Excessive use of laxatives, diuretics or enemas
* Periods of fasting
* Excessive exercise
* Secrecy and reluctance to socialise
* Shoplifting for food; abnormal amounts of money spent on food
* Food disappearing unexpectedly or being secretly hoarded
* Sore throat, tooth decay and bad breath caused by excessive vomiting
* Swollen salivary glands making the face rounder