I came in on an F2 Visa. Selective Services is requesting an i20 from me. Im now a green card holder and my 120 was taken by INS during acceptance. How do I go about retaining my I20?

That sounds complicated. Maybe talk to an immigration lawyer? (if the school can't help you... i would imagine they have great international person who knows everything there is to know about student visas - at i.e. center for internaional studies or the like)