Mexican reentering Canada as a tourist

My girlfriend is mexican and came to canada for the first time and stayed for 5 1/2 months (no visa, they just stamped her passport upon entry). She has gone back home and would like to come back again to see me and maybe take some adult education classes to better her english. Can she do the following:
1. can she reenter canada for the second time after being one month back home and if she can does she have to be back in her own country for more than one month before they allow her to come back? If she can come back now can she come for another 5 months or is it less the second time (ie. < than 6 months)
2. if no to my first question can she reenter if she enrolls in school (for < than 6 months)
3. if yes to my first question can she alternatively come back for > than 6 months and if so how? ie. by taking some school programs? If so, what kinds of application form does she need to fill out and what kind of schooling is accepted ie. part time adult education classes or full time in a community college?