American Heritage Dictionary - Cite This Source - Share This prac·tise (prāk'tĭs)
v. & n. Chiefly British
Variant of practice.
You can take a look at Dictionary.com when you have problems with words, it is a very useful site and I use it all the time.
I don't know the answer to the other question but I want to ask one as well for the teachers here. There are all sort of American variants to the British words. In a British exam, can I use American words in my compositions?