I am with Genghis here, isn't it the instructor/examiner that should initiate the go-around when performing a PFL. I remember during my training I was told time and time again that it was up to the examiner to initiate the go around by calling it. When I was an instructor this is what I always did, generally I wouldn't allow the student to go below 500', when I wanted them to go all the way to the ground then I knew the location of a couple of 'little' grass airfields that I would get them to set up for a PFL, always shocked the hell out of them when we got to 500' and I would just say 'continue to a landing thankyou'
Your big problem here will be regardless of the FAA regs, what does the school operating manual state. Even if the regs allow you below 500' in certain circumstances the school rules may not! If the school doesn't specify then I would just argue you were legal and that the training benefit by going below 500' was worthwhile in certain circumstances.
Good Luck