Each examinor who is testing at CPL and IR level is given there own call sign examXX.
Checked the UK AIP. ENR 1.6.2 (SSR procedures):
3.3 In accordance with ICAO Doc 8168 (PANS-OPS) Vol I, Part III, Section 3, Chapter 1, para 1.3, flight crew of aircraft equipped
with Mode S having an Aircraft Identification Feature shall set the Aircraft Identification in the transponder. This setting shall correspond
to the Aircraft Identification specified in item 7 of the ICAO flight plan, or if no flight plan has been filed, the aircraft registration.
Are those exams on a flight plan, with EXAM01 in item 7?
And IF you put it in the transponder, thus changing the default flight ID, should it not be the responsibility of the examiner/examinee to set the transponder back to its normal value (aircraft registration) after the exam? Now this responsibility falls on the unsuspecting next pilot. And I bet checking the flight ID is not part of the default checklist in a lot of places. (In fact, I bet most pilots would not even recognize a mode-S transponder for what it is, let alone be able to verify the Flight ID is correct.)