Food for though here for fosters ans saab....... you don't need a medical to flight instruct (to my knowledge) as long as the student has one and is PIC. Not having a medical makes your comm certificate invalid. Your CFI has to be current but that is about it. Also, you can instruct on a 3rd class medical which only give you private pilot privileges not comm. Caution ....... your entering the gray area of the FAA.