Squawk 7700.
That being said though, a stall is not an aerobatic manoeuvre and can legally be executed at any height. The 3,000ft or so that instructors use is a guide / best practice only.
The guide or best practice only of 3000 ft is certainly best practice for ripping off a student. For decades flying schools have perpetuated the myth that recovery from practice stalls must be completed by 3000 ft. It originated from Tiger Moth and Chipmunk days when the propellers of those early types were prone to stopping during aerobatics and stalling.
Because these aircraft were not equipped with starter motors, the only way to get a stationary prop to turn and start the engine was to dive steeply often to Vno. Thus the 3000 ft could be quickly used up in the dive leaving not much time and height to plan a forced landing if the engine failed to start. In fact part of the safety checks prior to stall practice was to ensure a forced landing area was within gliding distance.
Of course, the minimum height for practice stall recovery will vary with aircraft type but to flog a C152, C172 or a Warrior or a Jabiru all the way up to 3-4000 ft just to conduct stall recovery practice, is cheating an unsuspecting student out of lots dollars which ultimately puts the money into the instructors pocket and the flying school account.
One regional CASA's view (I have this on paper) is the minimum height for stall recovery practice is not a regulation. It is whatever height the flying school publishes in its Operations Manual. It can vary from school to school. But whatever restriction is published in the flying school operations manual is what CASA and its lawyer will hang you on. In Australia 3000 ft is the common figure and a common rip-off.