Read some books aimed at your target audience and see what kind of language the authors used. That's really the best way for finding out this sort of thing.
Since I'm from Australia, I always heard swearing ever since I could understand. Then it was worse when I was the youngest of my siblings, so they would start swearing when they were arguing with my mum's former partner. When I was twelve I was cursing off my little head but after I left that school I pretty much chilled down on the swearing which didn't come back until I was at least fourteen. But getting to the point, I think 'git' should be fine. But there's always one person that gets offended over something - even it's completely ridiculous. I've read stories set for preteens that say 'damn' or 'piss off'. Then again, I don't get so offended over swear words except for the constant use of the c-word.
This link might be of some help in the US, not sure about the relevance in other countries: Banned Books Week: A Book List It includes lists of books that were banned by public schools. Fascinating stuff.
E.g. in Half Bad, the author Sally Green does not use swear words, but instead says (for example) 'I shouted in anger and dropped some swear words in it'. In that book you can easily encounter the word 'shit' almost everywhere. And, for sure, in the last part, there's the F***! expression.
