If I was to do an arrangement of the Mozart Clarinet Concerto for a group of clarinets, would I be breaking the law? I will obviously give credit to Mozart, but is there anything else I have to do?

Just giving credit to Mozart is sufficient as he died sufficiently long ago .

70 years isn't it?

Good luck PD!

Indeed it is 70 years after the death of the composer that copyright lapses... Things get more complicated if you base your arrangement on a specific edition, because then the copyright of the editor has to be asserted.. Assuming though that the original score/edition is widely known and that is your basis then there's nothing else to worry about!

Yes that should be fine, as Mozart is very much in the public domain now! The only thing you must remember, which you seem well aware of, is to credit the composer so you are not seen as trying to pass their work as your own, otherwise known as plagarism. Good luck.

On an related matter, I was wondering the other day: if you, for instance, handed in a Wikipedia article for coursework or something in school, would it count as plagiarism? Because Wikipedia is 'copyleft' and the articles are a combination of many different people's work, so would it still count?

My understanding is anything you submit that is not your own work is plagarism. You can credit what is not your own work and what is (in pd's case, not Mozart's music, but her arrangement). Things don't even need to say 'copyright' to have this protection. Handing in an article as such would not be your own work, hence not acceptable. The only exeption is if you use it as an appendix or similar example to support your own work, and clearly give the source.

Plagarism whether intended or not usually results in automatic failure, so remember that!

Err yes, because you are passing off lots of people's work as your own. If you cite the Wikipedia article, fair enough...

Yeah, I was getting a bit muddled with copyright infringement. I wasn't thinking of trying it though