Too many pupils are being pushed out of mainstream classes in England and abandoned with an inadequate education, says a report from MPs.
The education select committee warns of a "Wild West" approach to the education provided for pupils who have been excluded from school.
There were also concerns about schools removing pupils to boost their position in league tables.
Committee chair Robert Halfon warned of too many "forgotten children".
He said schools showed a "lack of moral accountability" in what happened to pupils after they had been moved out of mainstream lessons.
The report raises concerns about the excessive use of both formal and informal exclusions.
It warned of so-called "off-rolling", in which difficult pupils were taken off a school's register and moved to another setting, such as a pupil referral unit or home education.
The MPs were told that this could be driven by schools wanting to have better exam statistics in performance tables.
But there were concerns that once pupils had been moved away from school, there was too little scrutiny of the quality of lessons.
Mr Halfon warned that those losing out were often from families with greatest need, such as those in poverty, children in care and children with special educational needs.
The committee is calling for a "bill of rights" for parents, giving them more information about their entitlements if pupils are removed from mainstream lessons.
It also calls for more incentives for schools to keep supporting challenging pupils.
There have been warnings about rising numbers of pupils being excluded from school.
Ofsted has written to head teachers raising concerns about high levels of pupils being removed from secondary schools.
Official figures published this month show a 15% increase in permanent exclusions.
The report from MPs suggested that as schools developed a "zero tolerance" approach to bad behaviour, more children were being excluded.
"We face the scandal of ever-increasing numbers of children being excluded and being left abandoned to a forgotten part of our education system which too often fails to deliver good outcomes for these young people," said Mr Halfon.
"Parents and pupils face a system which isn't designed for their needs, too often being left to a Wild West of exclusions," he said.
Geoff Barton, leader of the ASCL head teachers' union. said: "These are the young people on the margins, who are at most risk of severely reduced life chances, and we all have a moral responsibility to look after them."
The Children's Commissioner for England, Anne Longfield, said the report confirms warnings that "some schools are seeking to improve their overall exam results by removing some of their most vulnerable children from the school roll".
"Sadly, this often includes a high number with special education needs and disabilities, who have no option but to go into inappropriate alternative provision or home education because they are seen as "too difficult" or "too expensive" to teach," she said.
School Standards Minister Nick Gibb said: "The number of children being excluded is lower than it was 10 years ago, but exclusions should only ever be used as a last resort.
"The rules are clear, that they should always be reasonable and justified. Where pupils are excluded, the quality of education they receive should be no different than mainstream settings.
"We are taking a range of actions to drive up the quality of alternative provision, and have launched an external review to look at how exclusions are used, and why certain groups are disproportionally affected," said the minister.