I would be seriously unimpressed if a school did that - particularly showing a second horror film that's too old for the class (and the class above and some of the class above that!) having had the inappropriateness of the first film brought to their attention.
Definitely agree with sending a strongly worded complaint to the head, copying in the head of year and the chair of the governors.
DS1 is also 13 - he got to watch a film in school over a few lessons this week - but turns out it was a german film in his german lesson, they've had 10=15 minutes a lesson and used it as the basis of the lesson for vocab, listening to spoken German, trying to translate it etc. Also turns out it is a black and white film - not sure which one - but can't see it being a recent block buster or one that the dc would normally rush to watch...
ds2 is in y5 and we had to sign a permission slip a few months back because they wanted to show them an extract from the Fantastic Beasts dvd, they were doing a topic about descriptive writing, and comparing written descriptions of the beasts to some of the beasts in the film, then trying to write their own descriptions of some of the beasts they saw in the film but hadn't read about, before finding out how they had been described in writing.
Both those scenarios are very different to what your dd experienced.
I would ask the school for the risk assessment they carried out for showing two 15 rated horror films to a class of 12 and 13 year olds (assuming they are y8, even if there aren't any 12 year olds left in the class, they ought to risk assess it as if there are a few), with particular interest on justifying a senseless horror film being shown (whereas if say there was a version of a shakespeare film that was a 15 rating, that tied into the play they were doing in school, I can see why they would at least be considering it). And also what provision was made for those that didn't want to see it - both those that were brave enough to speak up and say this and those that felt too intimidated by the rest of the class to speak up and say they didn't want to see it. Also what provision they were making for those children that were suffering nightmares as a result of it, and what instructions were given to the children about talking about the film to younger children (whether in school or at home to younger siblings or friends).
I would also ask them what their current protocols are regarding watching films in school, both during the year as part of their studies (eg of a shakespeare play, Boy in Striped PJs, MFL etc as already mentioned) and those shown at the end of term recreationally. Include selection of film, age rating vs age of students, appropriateness, etc etc.
This is definitely something they need to have a policy on and stick to it!