The Department of Special Investigation (DSI) is scrutinising more than 1,000 schools in Bangkok over allegations of "tea money" being demanded for student places.
Deputy DSI chief Suchart Wong-ananchai said if any evidence is turned up, the case will be given special treatment.
He said demands for "tea money" had turned education into a money-making business, adding it was highly inappropriate, given the annual subsidies the government injects into state and private schools.
The "tea money" is often disguised by fancy names, such as financial support for school alumni or a school, he said.
"This practice is deep-rooted, but we have never talked about it. The general public has an idea of the amount of money prestigious schools demand. It is extortion of parents and discriminatory treatment against low-income families," Pol Col Suchart said.
The demands for "support" can be in the six or seven figure range and can total more than a billion baht per semester.
Students who find obstacles to certain schools being cleared by the payment of "tea money" tended to grow up with a money and profit fixation rather than a community-oriented attitude, he said.
Public schools in Bangkok are run by the Education Ministry and Bangkok Metropolitan Administration.
Deputy Bangkok governor Wallop Suwandee, who oversees education, said he had never received any complaints about "tea money" requests.
He insisted it did not occur in city-run schools, citing the BMA's policy of providing 12 years of free schooling.