Rumours of a sexual encounter between two pupils in a school emerged on Facebook, leading to an expulsion (REUTERS/Thierry Roge)

A grammar school has been left red-faced after it took back a pupil it had expelled purely on the basis of an unfounded Facebook rumour that he had had sex with another pupil in the school toilets.

Rumours surfaced on Facebook falsely claiming that one of West Kirby Grammar School's students, 17-year-old Trevor Evans, had sex with his girlfriend, also a pupil there, on school premises.

The pair were suspended by head teacher, Glenice Robinson, after staff discovered the rumours.Trevor was excluded and his girlfriend was asked to leave.

Evans's mother, Honora Evans, appealed against the decision and independent local authority adjudicators overturned the expulsion order.

The adjudicators said the school had not followed proper procedure to let the accused present written statements of defence and had not presented the alleged evidence against them.

Adjudicators also found that the school had acted unfairly by expelling Evans and letting his girlfriend leave.

Evans denies the pair did anything wrong.

"I just want to get back to school and resume my studies," he told the Daily Telegraph.

His mother said it was a "vindictive campaign hatched by some girls at the school who posted malicious rumours about him on Facebook".

Robinson would not comment on the specific incident but said the school "always acts in the best interests of pupils".