The first-ever convictions for operating an unregistered school in England have been imposed at Westminster Magistrates' Court.
Nacerdine Talbi and Beatrix Bernhardt faced charges over the Al-Istiqamah Learning Centre, which taught about 58 pupils in a west London office block.
The defendants had argued that this was a study centre where home-educated children had part-time tuition.
But the court ruled that they were in effect running a full-time school.
The chief magistrate of England and Wales, Emma Arbuthnot, in the first verdict of its kind, said the evidence showed the centre was "being operated as an unregistered independent educational institution providing full-time education".
The defendants have been given a community order of a 12-week night time curfew and the court heard that the school would be likely to be closed temporarily.
The institution itself, of which Mr Talbi was the director, was fined £100.
This landmark case followed warnings over several years from the education watchdog Ofsted that there could be hundreds of such unregulated institutions.
There had been calls for action against such schools, with inspectors saying that they were avoiding scrutiny over the quality of lessons and teachers and the safety of pupils.
The Al-Istiqamah Learning Centre had claimed that it was not a formal, full-time school.
It argued that pupils did not have more than 18 hours per week of lessons that, according to guidelines, could have made it liable to register as an independent school.
Ofsted inspectors had tried to visit on several occasions - and the court heard evidence that at least 27 pupils were at the school for 25 hours per week and others more than 18 hours.
There were other indications of this being a full-time school in all but name - with a head teacher, homework and fees charged in a way that suggested academic terms.
There were lessons in subjects including maths, science, geography, history, English and Arabic, with fees of £250 per month for secondary age pupils.
The hours were such that pupils could not have attended school elsewhere, said inspectors.
The chief magistrate concluded that the centre was "providing all, or substantially all," of the education for those attending for 25 hours.
As such the centre was "being operated as an unregistered independent educational institution providing full-time education".
Ofsted chief Amanda Spielman said: "We hope that today's judgment sends out a message to all those running such schools that they will face justice.
"We will continue to investigate and expose illegal, unregistered schools and where we find them, play our part in making sure they are closed or become properly registered."
Education Minister Lord Agnew welcomed the court's ruling.
"We have always been clear that where schools are operating illegally, action will be taken and this decision is evidence of that."
Lord Agnew said it would help to "keep children safe and ensure they are getting the best possible education".