Anthony Campen, 29, kidnapped the chicken from the school and took it to a house party where he posed with it before releasing it into a field. Police used the pictures to force Campen to admit his crime.
Teachers and parents at the Bishops' Church of England and Roman Catholic Primary School in Chelmsford, Essex, suspected fowl play when pet chicken, Olympia, went missing. They released a newsletter pleading for sightings but so far these have been unsuccessful.
Magistrates in Chelmsford were told of the distress caused to young children at the school who had lost a much-loved pet which they had raised from a chick as part of a school project.
Campen admitted causing unnecessary suffering to a protected animal and was ordered to pay £130 compensation and £85 costs.
Unrepentant Campen later tweeted: "£235 for a f*cking chicken. Right, i'm off for a kfc!!"
A spokesman for Essex Police said: "Essex Police are pleased that Campen has admitted this offence and that he has been ordered to pay compensation.
"This chicken had originally belonged to a primary school in Chelmsford and the loss of the bird caused great distress to the children.
"The chicken has never been found and it is highly unlikely it would have survived in the wild."