My i-20 has expired, but i don't want to have violation on my record. what should i do?

If the school did not report you as being in violation of status and you depart before February 20, there should be no problem. If you were reported to be in violation of status, you may not be able to obtain another visa for five years. Since the shool suggested you apply for reinstatement, it appears you have been in violation of status.