By Sean Coughlan
BBC News education reporter, at the ATL conference
Teenagers living in rural areas are using horse tranquilisers as a drug, says a teacher.
Children are not ready for work on Mondays
Elizabeth Greed, who teaches in Wiltshire, says pupils are taking the drugs at weekends.
"The feeling is don't expect these pupils to be co-operative on Mondays or Tuesdays," she told the Association of Teachers and Lecturers' conference.
Mrs Greed - a teacher for 35 years - also condemned poor classroom behaviour as a "social disease".
Taking the horse tranquilisers produced "euphoria and an inability to concentrate" in youngsters, she told delegates in Torquay.
Ketamine, a powerful anaesthetic used in human and horse surgery, is becoming more popular among drug users, the advice service Drugscope told BBC News.
Its side-effects include numbness, vomiting and unconsciousness, causing a possibility of choking.
Ketamine is available only on prescription.
Mrs Greed, 57, said parents were failing to instil a respect for teachers - and that schools often had to face parents "marching up to the school" over petty problems.
Many of these parents were afraid of being unpopular with their own children.
"It's a social disease. It's not just schools. It's a disease of a lack of respect. And that goes into every organisation and every institution."
She added: "I would challenge anyone to go into some of these secondary schools and cope with what we have to cope with. It's like sending us into the front line - and what can we do?"
Mrs Greed is clearly no shrinking violet. She told delegates that earlier in her career, she had disarmed a pupil who brought a Colt 45 to school and had also faced down a knife threat.
An ATL survey shows 72% of secondary school teachers are considering leaving teaching because they are exhausted by "persistent disruptive behaviour".
A Department for Education and Skills spokesman said: "Violence will not be tolerated, and we fully back headteachers' tough decisions to remove or prosecute anyone - whether parent or pupil - who is behaving in an aggressive way.
"We have given heads a wide range of tools to tackle drugs through education, detection and deterrence, up to and including drugs testing for pupils and 'one-strike' permanent exclusion for dealing in drugs."
The conference later voted in favour of a resolution demanding ministers give them more protection against violent and disruptive pupils plus "explicit national standards of acceptable pupil behaviour".
The conference also backed calls for full "risk assessments" to be undertaken before pupils with a history of violence are allowed into schools.