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Anyone can apply to post secondary if eligible. What one year post secondary program? She needs to show the fees and justify why she is only taking a one year program and show career progression and higher pay then she would receive by taking local program. She needs to show that she will return home since she is not immigrating to Canada. She is a student.
 
After 12 grade can my niece apply for study visa one year program as an international student?

If she is to trying and remain in Canada and hoping to secure PR this is a fairly risky option because she may not qualify for any or limited PGWP. Assume she doesn’t have a year of foreign work experience. Does she know French? You really need to research if it make sense to try to study in Canada right after high school and only do a 1 year program. Although many PR programs are changing you should be looking at all programs and whether your niece would have a chance of securing PR given current scores. It is extremely difficult to secure PR after a 1 year diploma.