Concerned parents of pupils at St Kentigern College wrote to the private Presbyterian school last year urging its principal to reconsider running the "very dark, brutal and gratuitous" musical Sweeney Todd as it had "no redeeming" features.
The revelation comes after two teenage boys' throats were cut on opening night of the production when a real razor wrapped in duct tap was used as a prop.
However, parents spoken to by the Herald were adamant the letter - dated September 17, 2015 - was not spurred by any concerns over health and safety at the show, but questions over how the gruesome musical aligned with the school's Christian values.
A copy of the letter - and the school's response - has been given to the Weekend Herald.
"Apart from the great music - there are no redeeming characters or themes in the musical and the key themes are revenge, gruesome deaths, rape and betrayal," the letter, signed by 28 parents, said.
They described it as "very dark, brutal, and gratuitous with no positive messaging", and "inappropriate" to be shown to a school community.
They questioned the moral message portrayed in the musical, and said it was the culmination of a trend towards productions of an adult nature that had sparked concerns among parents.
In response, St Kentigern head of college, Steve Cole, said the school was committed to going ahead with the production but would be "very mindful of the concerns you raised".
"The College Sweeney Todd production is the school edition of the musical and indeed has recently been performed by a number of New Zealand and Australian schools," it said.
Scenes deemed to be of a mature nature would be "staged in a way which takes into consideration the age of the performers without compromising the integrity of the show", Mr Cole said.
One father who signed the letter said there was "absolutely no concerns at all from a health and safety perspective", pointing to the "pretty dark" themes in Sweeney Todd and how it sat within a Christian school.
A St Kentigern College spokeswoman declined to comment on the letter, saying the incident was still under investigation.