Boring is the translation most people will give you for tsumaranai. This gets you into difficulty with the set gift-giving phrase 'Tsumaranai mono desu ga...' which does not mean 'This is just a boring thing.' It's a thing without much value, 'nothing much'.

Which is why I don't think the teacher was saying What a boring ambition to the kid: 'your dream fails to interest me'- and I don't think the sudden realization that he was an uninteresting person was what drove him to violence: Oh my god! I'm not interesting! I must kill this woman who's called me a bore!' Surely it's the realization of his life-long mediocrity and, well, failure to be anything much, that's the sore point?