Not sure there’s an obligation on the school to ask parental permission to teach your kids the curriculum on the basis that there’s stuff you might not want them to see or know. After all, where would the line be?
Also in Scotland and I’ve never been asked for AV consent since DS left primary. My DS is also in third year, he’s been 14 for two months, and is one of the youngest kids in his year. Some of his friends are already 15/soon will be.
In English and Media Studies they’re often shown 15 films, and recommended to watch films at home to expand their cinematic horizons. These are often 15, and occasionally 18 - in those circs the teacher uses the caveat ‘if you’re allowed by your parents.’
In History they use 15 cert sources from 3rd year onwards (pictorial resources/ documentaries). We recently visited a concentration camp and there were no under 14s permitted. Presumably educationalists have also looked into age appropriateness and susceptibility when constructing the current curriculum.
As other PPs have said, Psycho has been taught for years. Other texts set by schools for decades contain subjects which are more morally opaque but equally as upsetting as Psycho; there’s infanticide, suicide and murder galore in Shakespeare’s tragedies. Brighton Rock is about murder and coercive abuse - we read it in 4th year (I was 14 for six months of 4th year) To Kill a Mocking Bird has a rape, implications of child abuse/abduction and an attempted racist lynching. The reason these books/films are used is because they are well-written and created by masters of their art, but which come from an era where difficult subjects were handled without going into graphic detail.
It’s a shame Psycho upset your daughter, assuming that’s why you’ve complained. If it’s not her upset but your displeasure that’s prompted the complaint, I wonder if it’s because it’s a film about a murder?
If you deconstruct the Psycho murder scene film frame by frame (as DS has recently done for a film-making project) it’s actually very cleverly done; there’s no nudity and no injury shown, because the naked female form and wounding wouldn’t have got past the censors, so everything is implied. There’s also a strong moral message to the film (ie. theft is bad, no matter why you do it, murder is evil, the murderer is brought to justice and is not implied to be any sort anti-villain, as is often the case with modern murder literature (think Ripley, Hannibal). Thats not true of a lot of things teens watch/play/listen to.