Professor Munn, the country's leading researcher on discipline and exclusions, said headteachers should not exclude more pupils in a bid to meet new Government targets for exam passes. For the good of an inclusive society, Scotland should not follow the English path of rising exclusions.
Donald Dewar, the Scottish Secretary, chose the meeting in Edinburgh to launch the Government's guidance to heads on exclusions. It calls for an end to informal exclusions and fairer recording of pupils sent home.
But Professor Munn feared heads could be tempted: "There may be a risk of schools excluding pupils who make it difficult for them to reach targets. Raising attainment should not be at the expense of the most economically and socially disadvantaged."
Sometimes exclusion turned in the last hope some children had. School was a constant when many excluded pupils were coping with family breakdown, bereavement, abuse and unemployment.
In Scotland 5,000 children were excluded in eight months. "That seems to me quite a lot of children,'' Professor Munn said.
There was enormous variation between schools. Some excluded up to 10 per cent of their roll and one sent home 110 children. Others have no exclusions. Serious offences such as violence and use of drugs accounted for only a small number of cases.
"Informal exclusions are widespread, regardless of whether it is permitted by the authority or not,'' she said. Boys were four times as likely to be excluded. Their persistent misbehaviour, the "drip, drip, drip effect'', leads to exclusion.
Professor Munn admitted there was "no magic answer'', but underpinning it all was the school's ideology and beliefs.
Schools which wanted to teach only those pupils who wanted to learn were likely to have high exclusion rates. Those who took a more social welfare approach had lower rates. The views of the head and senior staff were crucial.
The Scottish Office guidance recommends all exclusions, however short, should be recorded. It warns that suspension or sending home could be against the law if regulations are not followed.
Mr Dewar told the conference: "It cannot be fair that in one authority a child can be 'informally' sent home with no record of it being kept, while somewhere else this is not allowed."