I have had to join this forum JUST so I can reply and, in some way, defend the teacher.
I am a teacher (of English and Media Studies) for 14-18 year olds. I also run the film club for years 7-9 (ages 11-14). I have also sent a letter to my students' parents regarding the watching of 15 films.
For the Media GCSE we watched Blair Witch project as part of a film unit I was doing, partly because I felt it was good example of cinematography, viral marketing campaigns, tension building etc. This was shown to year 10 pupils who would have been 15, or almost 15, and so ok for the content.
I have also shown some 15s to my film club. One example being 'Gremlins', which if classified now would probably be rated a 12 as it is so mild in the violence/gore/swearing that 15 is far too high (but obv 80s sensibilities were more strict). Compare this to the current film we are watching ('About a Boy' which is a 12) and it seems like a Disney film comparatively.
As for the showing of films underage, schools are not held to the classification but advised to use them as guidelines. This is so clips and films relevant to the syllabus can be used (e.g. we are currently studying 'action adventure' and I have used clips from Kill Bill among others as an illustartion of female heros which could subvert the traditional notion of male hero). It is the teacher's professional judgment whether it is suitable or relevant to the topic. Also teachers often use genres which they enjoy/feel comfortable with to be able to explore more confidently. The film choice may be questionable, but if it has an academic reason I woulnd't see the problem - it's not like they are watching 'Debbie Does Dallas' or ' Saw 3'.
And finally - Media Studies is NOT a 'soft' or 'easy' subject. It DOES NOT involve watching films all year long. On my course, the GCSE students will have watched 3 films in full - 2 as thier exam this year is on Action Adventure (and one of these films was on a study day), and one film for their coursework on film. Media studies GCSE (and A level) requires a lot of creativity and A LOT of coursework. They also need to have a good, in depth knowldge of the topic for the exam, understand some theories and be able to communicate clearly and effectively.
It is, admittedly, an exam that is hard to fail completely. BUt it is also a subject that is hard to excel in as it is such a wide subject area and the specification requires a lot to be done and a lot of knowledge to be shown in a very short amount of time.
I personally feel that Media is a very important subject in today's life as it has such a huge impact on everything we do (in fact Mumsnet itself is a fascinating use of New Media, convergence and developments in technology and society).
Thank you for reading my mini rant - and sorry for any typos - it's 11pm on SATURDAY night and I am 3/4 way through a bottle of Rose!