FARs would require the student to be PIC. JARs would require the instructor to be PIC. So which is it? There can only be one PIC at any one time.
Correct FFF......under FAR's both the student and the Instructor can log PIC.
The student because he/she is rated in the aircraft and the Instructor is PIC for having the final say in the outcome of the flight and ultimately responsible.
I assume the Instructor can log under JAA regulations while the student logs under FAA regulations.
Doesn't make whole lot of sense though if the student is being taught for a JAA rating.
But like I said earlier, the whole question has some serious regulatory and practical problems.
Like mentioned, it is far easier to give an FAA Instructor some standardization training then getting a JAA Instructor without FAA licenses/ratings.