My OPT is expiring this June and want to go back school to do PhD. Got admission for Fall 2018 and my concern is can I stay in the US after my OPT expires waiting for classes
You have grace period of 60 days usually to stay in the country after your OPT expires. If your school starts within 2 months, it should be fine, if NOT, you may have to leave and come back. I suggest you speak to the DSO at the new school and current school so that you have better clarity on if you need to leave and if there are any alternatives.