Suleyman Folami and Muanet Daniah had been found guilty of running the illegal school after a trial at Westminster Magistrates in March 2020.
The pair were each handed a four-week custodial sentence, suspended for a year.
They appealed the convictions but last week Southwark Crown Court rejected the appeal.
The judge found they were clearly trying to “bend the rules” and ordered them to pay additional costs of £640.
Ofsted officials had inspected Advance Education Centre in Brent three times between December 2018 and March 2019.
They found the ground floor of the building was being used for a legitimate primary school but the upper floor was an unregistered school for secondary pupils.
The pair claimed they were only offering a supplementary school.
But the court heard how staff worked between the schools, with as many as 66 children attending the illegal school for at least 25 hours a week.
The unregistered school set homework, issued detentions and it operated full-time.
Inspectors found several safeguarding risks at the school, including inadequate supervision of children and strangers walking unchallenged through the building.
Ofsted described the school as “chaotic” and said the adults in charge appeared to have significant difficulty managing pupils’ behaviour.
Amanda Spielman, Her Majesty’s Chief Inspector, said: “The people in charge of this setting were fully aware they were running an unregistered school, but they chose to break the law anyway.
“So I am very pleased the Crown Court has reinforced the verdict returned by the Magistrates’ Court last year.
“Unregistered schools deny children a proper education and put their well-being at risk.”
Ms Spielman said the government needed to take urgent action to close loopholes which allowed unregistered schools to continue running “on the edge of the law”.
“I am also concerned that individuals convicted of running unregistered schools may still be involved with children’s education in registered schools,” she said.
“This is not appropriate, and we are working with DfE and other authorities to take action on this.”