practice is the noun and practise is the verb.E G 'Go and do your piano practice, ' but 'she always practises the piano' once an English teacher always an English teacher!
Although the 'z' in word like organize, ostracize etc is current USA usage, it actually comes from the old English! It was us who changed to the 's', and the Americans kept the old version. So, if you're being purist about it....