I was allowed to watch (with an older friend) nightmare on elm street, the chucky doll (childsplay?)films, IT (about the clown) and jaws all at age 5-6. The nightmare on elm street films and "freddys nightmares" the tv series, i was allowed to Watch much like my own DS watches roary! As a parent i find my own mothers choices in this matter absolutely lazy and bloody ridiculous.
At the time child's play scared me so much I couldn't have any dolls anywhere near me, and the IT film instilled a lifelong fear of clowns. im with seeley booth on clowns, shoot the feckers!
Nightmare on elm street was ok until i suddenly realised what it was about, then had violent nightmares for months. Jaws I only watched once so I dont think it affected me as much, and I do have to confess its one of my favourites as an adult.
I do think my mum was a bit of a twat about it given the nature of the films, however that said I wouldnt stop my own child watching jurrassic park when he is a bit older / shows an interest as i feel that is natural history, murder based horror films are entirely unnatural and therefore reserved for adulthood.
I still get the creeps when i hear "one two freddys coming for you" 
I think in the OPs case it depends exactly what the children are permitted to view and with whom. I do agree that things are rated for a reason. DS is almost three I would let him watch jurrassic park but not jaws, and i can safely say films like
The ring, and the human centipede, hostel etc (true gory horror) I wouldnt let him watch until late teens as they genuinely scared me! Its about the levels of violence and understanding!
For example DP and I settled down to watch "legend" which is a PG film from our yoof with tom cruise about unicorns etc, as it was a pg and we both thought we remembered the plot we let DS start to watch... Thankfully he fell asleep! The plot is darker than either of us remembered and the main baddys helper BLIX (google him) is one scary fecker!! Just hows how we remember things!
As for taking material / items relating to a home experience to school Id be a bit pissed off if my child was shown something I considered inappropriate as my child may be more sensitive than the OPs, and shouldn't be at risk of exposure to materials I may not agree with in a safe environment, it shouldnt be something I as a parent have to worry about, so I think the teacher was not at all unreasonable.