I know 60 day grace period starts from the day OPT ends then when does that 90 day period start? From the day I-20 has been updated or based on OPT?

Let's say OPT expires in mid November then till mid Jan, it's 60 day grace period and if I get my I-20 in Jan first week then my 90 day starts from the day my I-20 has been updated or it starts based on OPT, like 90 day starts from mid Jan till min April? Because F-1 students have to attend classes within those 90 days right? If It becomes 91 days then what happens? or is that 90 days for session start date or we have to attend a class by then? I asked my school but they sound confused. I don't want to take a chance just because I didn't ask. Please let me know if you guys have any idea.