A permit for what? You hold an ICAO member commercial license. If you are at least 23 years old and have 1500 hours with all the breakdowns, you are eligible for a FAA ATP. Pass the written test, if you are going to take the checkride in an airplane get a FAA medical and train for and pass the checkride.
I'd say the flight school has not seen many foreign students. Every piece of FAA paper has a form number. Ask them what is the form number for the permit that they say you need.
Your 61.75 private certificate based on your Nicaraguan license does not matter. You'll keep that and as long as your Nicaraguan license is good it will be good.
Last edited by MarkerInbound; 30th November 2013 at 09:57.