American nursery schools expel children at three times the rate of primary and secondaries, according to a disturbing new report from Yale university.
Boys are four times more likely than girls to be excluded from nursery classes and blacks are expelled at twice the rate of whites and Hispanics, it added.
The study found that America's publicly-funded so-called "pre-school" programmes, attended by millions of three to five-year-olds as preparation for school, expelled children at a rate of 6.7 per 1,000, compared to 2.09 per 1,000 in primary and secondary schools.
The expulsion rate is especially troubling, with experts and reformers emphasising a strong nursery education as key to later school success, especially among low-income and minority students.
"In some cases, students are starting school having been kicked out of educational programmes several times," said the report's author Walter Gilliam, assistant professor of child psychiatry and psychology at Yale.
"When children have heard over and over again that they're an educational failure, that is a tough way to start a school career," he added.
A good start in nursery school means that children are less liable to have to repeat years, be placed in special education classes or drop out, and less likely to go to jail or claim welfare benefits in later life, according to research, Mr Gilliam said. "It changes their life trajectory."
But the findings reflect how far US nursery education falls short of what experts consider optimal - staff with masters degrees in early education teaching classes no bigger than seven, he added.
In typical programmes, staff, paid less than $10 (pound;5) an hour, with little more than a school-leaving certificate and "a few workshops" are in charge of 12 or more students, said Mr Gilliam and Nancy Freeman, assistant professor of early childhood education at the University of South Carolina.
"When children with challenging behaviour come along they're not prepared and so they expel them," said Ms Freeman. "The job interview in South Carolina is, 'do you have a pulse, no criminal record and can you read and write?'" she added. "It takes more education to be a dog groomer."
The largely white female workforce is ill-equipped to handle minorities with "different interaction styles" and "rambunctious" boys, Ms Freeman said.
Expulsions were fewer at primary school-based or US government-funded sites with better-trained teachers and more educational resources than at programmes contracted out to private providers, and halved where staff had access to psychologists, the study found.