Exclusion is often the only answer to persistent classroom disruption because of the lack of available alternatives, Barbara Clark, president of the Scottish Secondary Teachers' Association, said. Otherwise the "steady drip, drip effect" of disruption hampered the work of other pupils and teachers.
Mrs Clark warned: "There are waiting lists for everything and a child on a waiting list is a child being neglected.
"Exclusion often becomes the only real sanction that can be applied. But the problem with that is that youngsters get on to an exclusion merry-go-round. They are out for one week, two weeks, three weeks.
"They come back without any experience of education. They are back on Monday morning and is it any wonder they are back out on Monday afternoon?
"Eventually these children leave school with such a negative view of the education system that they pass this on to their own children and history repeats itself."
Such pupils often ended up in residential care yet a school near her in the Falkirk area had cut discipline problems by 50 per cent by allocating an assistant rector as a trouble-shooter.
Equality of educational opportunity for all meant increased support to deal with such pupils.
May Ferries, vice-president of the Educational Institute of Scotland, said: "It is about money, investment and smaller class sizes. Exclusion is not ideal but for a lot of us it is the only option."