almondbaby,
  i was in ur situation a year ago. After your opt expires you do have 90 days to get into school. So you should be fine. Just to be on the safe side i would suggest you tell your employer to do a premium processing which will get you your results in 2 weeks (if there is no rfe) and if they (uscis) send rfe, they will stop your premium process and start it again once you have submitted your docs.

hope this helps and good luck