We regularly show films of above the appropriate age (eg. Polanski's Macbeth for Year 9 SATs/McKellen's Richard III, also for SATs - both 15).
I also teach a unit for Year 8 based on the novel 'Private Peaceful' about WWI - I use (carefully chosen) extracts from 'The Monocled Mutineer', a BBC series rated 15.
Oh & extracts from assorted horror films (15 & 18) before I do GCSE coursework based on Edgar Allen Poe.
I send a letter home if I intend to use a film not rated for the age group. Frankly, I do so to protect myself rather than to protect the kids - I'm constantly shocked at what they download without their parents' knowledge ( or WITH their parents' knowledge, come to that).
I do try to be discreet when students' parents refuse permission. However, I don't watch 15s with them for a laugh! It's because a particular film will benefit their studies.
Also, a great many films that we use in English are quite old & wouldn't attract such a restrictive certificate these days.
Then again, I also teach a unit to Year 8 in which we watch bits of 'Jaws.' Scares the bejasus out of me. 12 Cert in the UK. Anything from PG > 18 elsewhere in the world - in fact, I get them to write an essay on what cert. they'd give it & why...usually they reckon 15!
Anyway. I (personally) think that young adults could do with a bit of thought-provoking, upsetting cinema such as Amistad. However, they should have asked your permission first - so write & complain by all means.