Holli McCann, 11, had tucked into the 20p confectionary on the first night of the week-long break in the Isle of Wight, along with two other Year Six roommates.
She described the secret snack in her first letter home, which was read by teachers despite being in a sealed envelope.
They immediately searched her room at Beaufort House hotel in Sandown for more chocolate, removing the lining of her suitcase and tipping out her toilet bag as if she was “running an international drug smuggling operation”.
When they found more bars, Holli was thrown off the holiday for breaching a “charter” banning chocolate.
A message was sent to her mother, Kerri, asking her to urgently contact the hotel.
Yvonne Graves, the headmistress at Bromet Primary School in Watford, Herts, told her what had happened and requested she come and collect her daughter immediately.
Holli’s mother urged Ms Graves to reconsider but she refused and said if Holli was not picked up she would have to attend all the planned activities but would not be allowed to join in any of them.
As a result, Mrs McCann had to drive through the night from her home in Croxley Green, Herts, to make the 160-mile round trip with two ferry crossings to collect Holli.
The unemployed 47-year-old, who is a full-time carer for her autistic son, had saved for six months to pay for the £300 holiday and was forced to borrow another £130 from family and friends to cover her own travel costs.
She said Holli had been “so excited” about the trip, which began on July 1, and was delighted to get a room with her best friends.
“They had been planning the feast weeks before the trip and Holli was in charge of bringing the chocolate,” she said.
“It wasn’t even at midnight. They ate the chocolate at about 9.30pm and it only went on for about 15 minutes. It’s not like they were having a party or making noise.
“The teachers had no idea about it until they read Holli’s letter to me.
“I am furious that they read her letter, it is like being in prison. It’s not like she is five – she is 11 and deserves privacy in what she writes to her mum.”
“Holli said she was really upset because they emptied her toiletry bag into the sink and pulled out the lining in her suitcase.
“It was carried out in such a manner you would have thought they were running an international drug smuggling operation from their hotel room.
“I don’t see how eating chocolate makes the holiday unsafe. They were not being naughty – they were just having fun.”
She said she thought it “too cruel” for her daughter to stay on the trip and not be able to participate in any activities, and has formally complained to the school and governors.
The school refused to comment on the row but Hertfordshire County Council said: “Before the Year Six trip to the Isle of Wight, parents and pupils were asked to sign a behaviour charter which clearly outlines how pupils should behave during the trip.
“This is to ensure that everyone can have a safe and enjoyable holiday.
“It was made clear that breaking any of the rules within the charter would result in parents being asked to take their child home, as was the case with this pupil.”