Like most here have said, descend to 2000' when established on a published segment of the appraoch that has a minimum altitude of at least 2000'. If the instructor cleared him direct to the FAF, an maintain 2500' he would not have the option of descending until passing the FAF. In this case the prudent option would be to refuse the approach clearance unless the controller can provide a lower altitude or a longer lineup so as to get established on a published segment earlier. The reason I emphasize the published segment part is that I used to go to Leon (Spain, if I remember correctly), there was a holding pattern on the OM aligned with the final course, but no extended final and no DME, and there always was a discussion if you were being vectored in above the OM crossing alt when you could start your descend (we weren't allowed to use the ILS, just the OM/NDB). I would convert the max holding speed to 1 minute ground distance, and use the GPS to start my descend. As we weren't approved for GPS approaches, only arrival, I am sure that although it worked, it wasn't necessarily legal.....
If you would dive and drive the NDB approach you would be 5 miles out at 400' AGL!!