Many part time jobs are 25 hours per week. But according to the rules a student can work upto 20 hours per week. Is it OK if I work for 25 hours per week instead of 20 hours?
No.
And it is not true that "many part-time jobs are 25 hours". It all depends on the case. it could be 30, 25, 20, 16, 15, 14, 10 and 8- ANY amount of hours.
According to the rules, working over 20 hours/week during the academic semesters is a violation of your study permit. If you are found to have not followed the rules of your visa, then you can pretty much forget getting any additional immigration benefits from CIC, even if they do not actively hunt you down and deport you