Pupils have expressed disappointment that their school will face no investigation over a data protection blunder that exposed the personal details of hundreds of their classmates.

Hampstead School reported itself to the Information Commissioner’s Office (ICO) last week after accidentally making the details of more than 400 pupils accessible to every student at the school in Westbere Road, Cricklewood.

But the data protection watchdog has decided the matter was not serious enough to warrant a full probe.

A spokesman said: “On the basis of the details available to us, we have no plans to take further action.”

The error, which was the result of a botched mailshot, allowed a spreadsheet containing the names of pupils and their parents’ names, addresses, phone numbers and, in some cases, emails to be left on a shared drive for nearly 18 months before anyone realised.

It only came to light after the file was discovered by pupils who write The Hampstead Trash, a satirical blog about the secondary school.

In a statement, the anonymous students behind The Hampstead Trash, all currently at the school, described it as “unfortunate” that the ICO “does not take the data breach seriously enough to pursue any line of inquiry”.

The statement added: “However, we are thankful that no fine was imposed on the school, as this would have detracted from much needed funds.

“We have implored the school to apologise to all students and assure them that an incident such as this will never occur again.”