The law governing children who are taught at home needs to change to ensure they are getting a proper education, teachers say.
There are no official figures on the numbers educated outside school
It is estimated that many thousands learn outside mainstream schools, but parents do not have to let officials know what they are studying.
The Professional Association of Teachers called for more monitoring.
But most children had "happy experiences" of home education, its annual conference in Bournemouth heard.
Bullying, exam-related stress, and parents' fear of being jailed or fined up to £1,000 for allowing their children to truant were said to be driving its growth,
Sometimes families wanted a "closer relationship" with their offspring, had religious or cultural objections to mainstream schools, or were waiting for places in the primary or secondary school of their choice, it was said.
The PAT heard home-educated children were the only ones to be taught by unqualified people, yet they were also the only group not consistently monitored.
Kim Tomsett, a monitor for West Sussex local education authority (LEA), said it should be compulsory for parents who chose to educate their children at home to ensure they were present during official visits.
At the moment, they have the legal right to refuse a home visit or, if they do consent, not to have the child present, even though the LEA has a legal duty to check up on them.
They can send in samples of the child's work, but Mrs Tomsett said it was difficult to tell from these whether parents were fulfilling their obligation, under the 1944 Education Act, to teach them up to their "age, aptitude and ability".
If the child has never been enrolled in mainstream education, their parent does not have to tell the LEA of their existence so there are no checks.
Despite the opposition of one PAT member who educated her children at home, delegates at the conference overwhelmingly backed Mrs Tomsett's demand for action.
Mrs Tomsett warned that a few of those ostensibly educated at home were being exploited as under-age workers.
She knew of one 15-year-old girl who had been working in a nursing home when she was supposed to be receiving tuition.
She had discovered this when she phoned asking to speak to the person in charge - and had recognised the girl's voice when she answered the phone.
Mrs Tomsett told delegates: "She said she was in charge. I asked her who else was there. She said, 'The cook and the cleaner'."
Advice to parents
Official attitudes to home educators vary.
In Scotland, the executive's Parentzone website says, at the top of the section on "choosing a school": "Parents are legally responsible for ensuring that their children are educated and normally fulfil this duty by sending their child to school."
It then adds: "Some parents choose to fulfil this duty by other means, such as education at home."
In England, the equivalent Department for Education and Skills website does not mention the option - even under the heading of what people can do if they cannot get a place at their preferred school.
It is under a section on learning outside the school environment, covering "topics such as computers, libraries and homework".
But a number of people e-mailing BBC News Online recently on the issue of school choice said they had decided, in frustration, to educate their children themselves.
There was a row recently when the head of the Association of Education Welfare Managers, Jenny Price, said some parents claimed to be educating their children at home to hide the fact they were abusing them
Having had complaints, Mrs Price said it was clear some education authorities "do not fully understand the home education ethos".
Talks were set up with the biggest organisation representing home educators, Education Otherwise.
A Department for Education and Skills spokesman said: "We believe that for most children school is the right place in which to receive education. However parents have the right to educate their children at home if they so wish.
"Where that happens the parents must take responsibility for ensuring that the education provided is suitable and full-time (for children of compulsory school age)."