This is the number one reason I don't want to go to university, so I totally understand your annoyance. Apart from standing up in front of the class with a megaphone and telling them to shut their mouths, there isn't much you can do to make people do what they don't want to... I know when I get overwhelmed by noises I plug my ears (it doesn't make it much quieter, but it does help with the "hum" of the background noise when you're trying to concentrate on one thing). You could also try bringing a camera or audio recorder to class so you can go back and listen for things you've missed. Good luck with your schooling!