Personally I would have to say that your teacher has a very limited way of looking at things. The reason I love the GF trilogy is cus it's intelligent and is not afraid to be complicated or deep in terms of themes. The novel in particular is more detailed than the movie which, itself is complicated and is worth seeing at least two or three times until you get a solid idea of what's going on.

Personally, I think perhaps your teacher hasn't read the book and doesn't realize that it's not about people being whacked, it's about alot more. Such as how our environment shape us and vica versa, love betrayal, intrigue, Damn, there is something poetic about it that even Shakespeare would have approved of.