"Wanna" can, in conversation, be an extremely casual spoken substitute for either "want a" or "want to". "I wanna watch" can either mean "I want a [wrist] watch" or "I want to watch [something]". If "want to" is the meaning intended, then "wanna" can stand anywhere that the more formal phrase can. Except in direct speech, such contractions as "wanna", "gonna", "coulda", "shoulda", etc, have no place in written English that aims for any level of formality above street-talk, and this should, in my opinion, be your concern as a teacher.
