Excluded teenagers who receive a minimal amount of home tuition are falling into a life of crime and drugs, a behaviour expert has warned.
Sir Alan Steer, who advised the previous government on improving discipline in schools in England, said the situation was a national scandal.
He told MPs a minimum standard for the teaching of excluded pupils was needed.
Sir Alan also said children with mental health problems were having to wait too long to receive professional help.
Giving evidence to the cross-party education select committee for its inquiry into behaviour and discipline in schools in England, Sir Alan said excluded youngsters were not properly monitored.
"We have children who are out of school who are receiving as little as an hour a week of home tuition, week after week, month after month.
"I would bet some of these 11, 12, 13-year-olds go into drugs, go into crime, go into prostitution - I'd be amazed if that wasn't true... that to me is a scandal."
Sir Alan told the MPs that children who had mental health problems were often let down by the system.
"It hovers round a national scandal - the issues of children's mental health," he said.
"We have come to this issue quite belatedly. As a young head teacher, it was extremely difficult to get medical acceptance of children having mental health problems."
Sir Alan said the worst case he knew of was when a child had to wait 18 months from being referred to a specialist to being seen.
The norm was a nine-month wait, he added, and there was enormous variation across the country.
He also raised concerns that summer-born children were much more likely to be considered to have special educational needs than those born in the autumn.
And he stressed that consistency of educational experience made a huge difference to society's most vulnerable children.
"Really high-quality classroom experiences are, in my book, one of the biggest equal opportunities you can ever find because those are the children who are most vulnerable if they get a weak experience.
Sir Alan was formerly head teacher of Seven Kings High School in the London Borough of Redbridge.
He investigated issues of school behaviour and discipline for the Labour administration.