The board of governors of a secondary school in Somerset has refused to expel a group of white pupils who reportedly chained a black student to a lamppost, subjected him to racist abuse and whipped him with sticks as part of a “mock slave auction”.

Police are investigating the incident, which took place during lunchtime on 22 January at a school in Bath. It is understood that at least seven teenagers participated in the alleged attack.

The headteacher of the school, which has not been named to protect the victim’s identity, initially expelled three of the alleged perpetrators, but school governors overturned the decision and allowed the teenagers to return to class. Four other pupils were temporarily excluded.

The school did not inform parents about the incident until Tuesday, and many have contacted the Bath Chronicle to express their anger.

“It just sends the wrong message out to the children. I’ve got a mixed-race son. He’s thinking ‘they’re going to stick up for the white kids but we get in trouble if we do something wrong’,” one father told the newspaper.

His partner added: “The boy it happened to is just the sweetest little thing ever. He’s so sweet and kind and gentle. It just makes it even worse. The thing is, I’ve got a mixed-race child and so has my partner. They believe nothing will happen to white children if they behave badly but something happens to them.”

In a message to parents and staff on Tuesday morning, the school said: “We have today become aware of media interest in an incident which took place in January and involved a group of established friends and related to a single incident of unacceptable behaviour within the school grounds.

“A full investigation was instigated in line with both internal school procedures and Department for Education requirements, including contact with the police. You can be assured that the school has taken this incident exceptionally seriously and that our absolute priority was, and remains, that the right path is taken for all those involved as well as the wider school community.

“Given the ongoing police investigation and the need to protect all those involved, you will understand that it has been necessary to maintain confidentiality and consequently that the school does not wish to comment further.”

A police spokesman said: “We were made aware of an incident at [the school] on 22 January, which took place around lunchtime. Inquiries are ongoing and the school is assisting. Six boys have voluntarily attended a police station and another has agreed to voluntarily attend.”