Poole Borough Council has defended its use of controversial laws to spy on a mother and her family 21 times in three weeks, saying it only invaded their privacy "minimally".
The authority also used Regulation of Investigatory Powers Act (Ripa) legislation on two other occasions to determine whether families were living in the right school catchment areas, a landmark hearing was told.
Mother-of-three Jenny Paton had applied for Lilliput First School in Poole, which was "educational gold dust", Ben Hooper, counsel for the district's borough council, said.
Ms Paton, 40, branded the authority "ludicrous and completely outrageous" as she took the authority to court for its use of Ripa legislation.
In the second day's evidence before the Investigatory Powers Tribunal in central London, Mr Hooper told the panel: "It was minimally invasive of privacy."
He said: It was a very successful state school where in June of this year it reported as being outstanding in every one of the areas it had been examined."
He said the authority had used Ripa powers on three occasions, adding: "This is only the third action in which this power has been used by the respondent on school admission.
"There are 3,600 school places to allocate each year in the borough... and this is the third time ever that this has been available."
Ripa, dubbed a "snooper's charter", was introduced in 2000 to give the police, security services and Revenue and Customs the powers to spy on people in the fight against crime and terrorism.
But it has been used to monitor relatively trivial offences by some local councils.