I applied for permanent resident in CEC class in January. I want to go back to school in May as part time student. Is it possible to go to school on open work permit or i have to wait for permanent residency? Please help me

You could not attend academic programs which are subject to the requirement of a study permit (i.e., a work permit does not automatically permit study in Canada). There are certain types of short term education which are not subject to study permit requirements, however. Finally, you do have the option of pursuing a study permit as well.