A teacher was knocked unconscious when a pupil dropped a full can of Coke on to her head from four storeys high.
But despite the fact she was rushed to hospital, the school asked police NOT to investigate the incident.
History teacher Jane Breeze was taken to the Royal Gwent Hospital, Newport, with head injuries at Croesyceiliog Comprehensive School in Cwmbran, last Friday.
The school contacted police to say they would 'deal with the incident' but Mrs Breeze – who is recovering at home – has now made a formal complaint after her MP described the school's decision as 'outrageous'.
David Davies MP contacted Gwent Police and Crime Commissioner Ian Johnston over a Welsh Government-backed guidance document aimed at not criminalising young people.
A senior police officer sent an email to Mr Davies - which the MP shared with Wales Online - saying: "I have personally spoken with Mrs Breeze this morning and wanted to update you regarding the action that we have taken on behalf of your constituent.
"The incident in question has been recorded as a crime by us, however the initial direction from the school was that they would deal with the incident under the School Crime Beat Protocol (that is, that they would take primacy for the investigation and subsequent outcome within the school discipline procedure, and that the police would take no formal action).
"Having spoken with Mrs Breeze this morning however, she would like formal action from the police."
The School Crime Beat Protocol, backed by the Welsh Government and the four Welsh police forces, says: "The aim of the... protocol is to avoid criminalising young people."
Mr Davies said: "I think it is outrageous that when a call was made to the police in this serious case of assault which resulted in a teacher suffering head injuries and being taken to hospital having been knocked unconscious, it was decided that the matter would be dealt with by the school and that there should be no police investigation.
"I'm very concerned that this decision appears to have been taken as a result of some Welsh Government guidance aimed at not criminalising young people.
"I can understand the principle of not involving the police if a pupil has nicked a bag of crisps or got into an argy-bargy in the school yard, but not reporting such a serious incident for investigation is, in my view, irresponsible and wrong.
"It sends out entirely the wrong message about an assault which could have had fatal consequences.
"It shouldn't have been left to the teacher herself or me as her MP to contact the police and call for an inquiry.
"The Welsh Government should look at its guidance again and issue a revised version."
A Welsh Government spokesman said: "This is a matter for the school and local authority to comment on. It would not be appropriate for us to comment."
Colin Crick, chair of the school governors, said: "The school contacted the police on Friday morning regarding the incident which was took place at 8.35am and was logged by the police at 9.32am.
"The acting head spoke to the teacher on the day to provide advice and to offer our full support."
A police spokesman said: "Gwent Police was called to Croesyceiliog Comprehensive School to a report of a teacher being injured by a falling object.
"An officer attended and Gwent Police is currently investigating an allegation of assault.
"No arrests have been made, and as the investigation is ongoing it would be inappropriate to comment further at this time."