Recently my H1B was rejected after I got a RFE for my petition and since I am not in OPT anymore I have enrolled in a school. My questions are as below:
Am I legally allowed to stay in the country now that I have a new I20?
How long do I need to stay out of the country to activate my new SEVIS?
Do I need to book an interview with the consulate?
Am I legally allowed to stay in the country now that I have a new I20?
Yes, if the course has already started. I assume it would have as you mention that you are already enrolled.
How long do I need to stay out of the country to activate my new SEVIS?
Your sevis would have been activated, if the course has already started by now.
Do I need to book an interview with the consulate?
Visa stamping only applies if you plan travel outside USA in near future.
Contact your international office of the school you have enrolled, immediately. They are best guide for you, for now on visa & sevis status. I would follow their advise.