An investigation is under way after a college sent the personal details of more than 600 potential students and their parents to fellow applicants.
Scarborough Sixth Form College included addresses, names and phone numbers in an email sent out on Monday.
Principal Marcus Towse has apologised for the "regrettable incident", but parents say it should not have happened.
The Information Commissioner's Office said it is "making enquiries".
The personal details were contained in a spreadsheet attached to an email sent to hundreds of applicants.
Later the same day a second email alerting people to the breach was sent apologising for the "inadvertent" error before asking those who had received the spreadsheet to delete it.
It went on to say the issue would be referred to the Information Commissioner and the college would "review urgently our processes" to prevent future data breaches.
One parent who received the email said it was "madness" this could happen so soon after new data protection laws were introduced.
The new laws mean businesses, including schools and colleges, have to meet higher standards for keeping data safe.
The parent said she was told the email, which was about a summer messaging service for new starters, should not have contained any attachments.
The woman, who did not want to be named, said the college had also handled things very badly, adding that "they should have contacted people by phone" to discuss the matter.
"Some parents might still be totally unaware," she said.
Apologising for the error, Mr Towse said the college had notified those affected by the breach and said it would now review its processes to avoid future errors.