Chambers English Dictionary
Collins English Dictionary
Oxford English Dictionary
All three give practice as a noun, practise as the verb.
Although you are able to use practice along with practicing without any fear of being incorrect
Please remember that I'm from the UK and do not use US English but British English, which can sometimes be a cause of confusion when we write. This is why I spell 'colour' like I do, no matter who i'm writing for
Edit: It seems as if it is a US/British English peculiarity:
http://www.askoxford.com/concise_oed/practice?view=uk