I'm not an American citizen but I was here as a student. I'm Italian and my international driver's permit expires in April. I came here in April 2015 and for 20 days I'm uncovered. What happens to me if I'll be stopped and my IDP is expired?
See How can I determine if I need an International Driving Permit? - it's generally only required when the script is different to yours.
Source: NZ citizen, and have used a NZ license without an IDP in the USA to rent and drive cars on several occasions, and have even been checked by police.