School inspectors have warned that there are hundreds of unregistered schools but that they have few powers to investigate or shut them down. When you think of a place that teaches groups of children - all day, every day - the word most people would use is "school".
But the law in England is surprisingly vague on what a school actually is. More than 350 sites - suspected of being unregistered schools - have been investigated in England by education watchdog Ofsted since a specialist taskforce was set up two years ago.
Ofsted inspectors say they don't have proper powers to inspect or close them. The people running these sites say they are not schools at all. Messy looking desk in scruffy room, photographed by Ofsted in unregistered schoolImage copyrightOFSTED
Some of them are on industrial estates, in church basements and even in pubs. The BBC has obtained pictures showing squalor, appalling food hygiene and dangerous wiring.
Some premises have blocked fire escapes and no access to clean drinking water. Many sites do no criminal records checking of staff.
But because of the confusing legal situation, most remain open.
Suri's son is about to leave his primary and go to big school. Not every parent is happy about the choice of education they end up with, but Suri - not her real name - breaks down in tears when she thinks about where her boy is going.
Later this year he will start at a "yeshiva" in the Haredi ultra-orthodox Jewish community in Stamford Hill, north London. It is not registered and its premises and teaching go unmonitored by the authorities.
Suri's son will never do GCSEs there, as these yeshivas usually don't provide any maths, English or science despite operating full-time. Suri, seen in silhouette, looking over Hackney
Suri believes her son's potential will not be realised Instead they offer a wholly religious education for boys, focused on scripture, from the age of 13.
At their schools, girls are often able to study secular subjects and do exams, but after a Haredi boy's Bar Mitzvah, devotion to religious education is imperative. Those who run yeshivas would argue that they are not schools and therefore don't have to abide by the rules that the state imposes.
If you have any information or experience with unregistered schools that you are willing to share with the BBC you can get in touch with us, in confidence, at haveyoursay@bbc.co.uk
And the Department for Education seems to accept that yeshivas are not technically schools. In a statement, the DfE told the BBC that yeshivas "do not meet the definition of a school and cannot therefore be prosecuted for operating as an unregistered school".
The DfE guidelines for England say that any place that teaches five or more children for at least 18 hours a week needs to be registered as a school. In Wales, anywhere offering "full-time education" is expected to register. In Northern Ireland the law requires all independent schools to register. The same applies in Scotland.
But legal guidance known to have been cited by the DfE says a site is not technically a school if it doesn't teach general classes of education, such as maths, English or science. Hence any place that only teaches religious education - be it Muslim, Jewish, Christian or any other faith - is arguably not a school.
From same link :The tearful motherSuri's son is about to leave his primary and go to big school.Not every parent is happy about the choice of education they end up with, but Suri - not her real name - breaks down in tears when she thinks about where