Parents are being forced to deal with an exclusion system that is like the "Wild West" and favours schools rather than pupils, MPs have warned.
In a highly critical report, the House of Commons Education Committee raised concerns about a "lack of moral accountability" among many schools which have little incentive to keep on students that are seen as "difficult or challenging".
It warned of an "alarming" increase in "hidden" exclusions - when pupils are unofficially removed from lessons.
And it concluded that excluded pupils, who may be put into different forms of education, known as alternative provision (AP), are being failed by England's education system.
Ministers have announced a review of the exclusions in England's state schools.
Figures published last week showed a hike in exclusions, with more than 40 children a day expelled and over 2,000 suspended from England's state schools in 2016-17.
The committee's report says: "Mainstream schools should be bastions of inclusion, and intentionally or not, this is not true of all mainstream schools.
"We have also seen an alarming increase in 'hidden' exclusions. The school environment means that schools are struggling to support pupils in their schools, which is then putting pressure on alternative providers.
"Pupils, parents and schools can end up in conflict, putting further pressure onto a system that should be supporting all pupils to achieve."
The cross-party group of MPs called for a "bill of rights", for pupils and their parents, which it said should include a commitment that schools do not rush to exclude pupils, and that they publish their exclusions rates each term.
"Parents deserve more information when their children are excluded: The exclusions process is currently weighted in favour of schools and leaves parents and pupils fighting a system that should be supporting them," the report says.
Committee chair Robert Halfon said: "Today, 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."
He added: "Parents and pupils face a system which isn't designed for their needs, too often being left to a Wild West of exclusions with too many pupils in alternative provision who shouldn't be there, and those who are there not receiving the right support or the early intervention needed to make a difference to their lives.
"We need a Bill of Rights for parents and pupils who access alternative provision and they deserve someone in their corner to be their champion during the often-difficult process of trying to get the best possible support."
There are at least 48,000 pupils who are educated outside of mainstream and special schools during the year, the report says, excluding those that remain on the full register of their school.
Not all youngsters who are in alternative provision (AP) have been excluded, for example, they may be there for medical reasons or those whose parents have been encouraged to take their child out of school voluntarily.
Examining the reasons for a rise in exclusions and AP referrals, the report says there is evidence of a rise in "zero-tolerance" behaviour policies, which may be creating "school environments where pupils are punished and ultimately excluded for incidents that could and should be managed within the mainstream school environment".
The committee also noted that there has been a rise in mental health needs in schools, as well as raising concerns about "off-rolling" – where pupils are taken off the school register, and the impact of government policies, such as reforms to school performance measures.
"An unfortunate and unintended consequence of the government's strong focus on school standards has led to school environments and practices that have resulted in disadvantaged children being disproportionately excluded, which includes a curriculum with a lack of focus on developing pupils' social and economic capital," it said.
"There appears to be a lack of moral accountability on the part of many schools and no incentive to, or deterrent to not, retain pupils who could be classed as difficult or challenging."
The report also concluded: "The quality of alternative provision is far too variable, with some outstanding provision in places and in others far too poor.
"The teachers, who play the crucial role in the education of pupils, can similarly be of high quality, while in other cases they are not. Even the best teachers may be lacking in suitable training and development, which impacts on the support that children receive.
"There seems to be high-quality AP despite the system, not because of it. There needs to be more collaboration between mainstream schools and AP settings - and we encourage schools and local areas to do this."
Geoff Barton, general secretary of the Association of School and College Leaders, said: "We have to do better for young people who are having problems in mainstream schools and are either at risk of exclusion or who need alternative provision.
"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.
"We are extremely concerned that school exclusions have risen as a direct result of government underfunding.
"Real-terms cuts have meant that schools are less able to provide the early intervention and support that is needed to prevent behaviour problems escalating to the point of exclusion. We also need a national strategy and the necessary funding to ensure high-quality alternative provision is available consistently across the country."
