Last year, a special unit set up to monitor private schools more closely recorded the highest number of unregistered institutions seen since the Council for Private Education (CPE) was set up.
The statutory board, which oversees the private education sector, told The Straits Times that it had nine cases - involving different unregistered schools - up from the four cases recorded in 2014.
This comes after CPE set up a department in 2014 to focus solely on monitoring and enforcement work, instead of also handling school registration processes.
The task of the five-man outfit is to spot potential issues before they become actual infringements.
It does so through various means including doing spot checks, acting on feedback and collecting evidence from school staff and students.
This allows CPE to send early warnings and advisories to remind schools of regulations they need to abide by, as well as take prosecution action depending on the severity of the charges.
Mr Remy Choo, the director in charge of CPE's regulatory departments, said there was "a small but growing number of private education institutions as well as non-registered entities engaging in practices that could mislead students and the public".
These include running diploma courses or even offering such courses on their websites without registering with CPE.
The four schools identified in 2014 were issued warnings as they had provided diploma programmes without being registered.
Three of the schools identified last year were also warned. The other six are still being investigated.
CPE said some of the schools complied by registering with it, while others returned to being short-term course providers.
The private education sector has undergone a shake-up since CPE was set up in 2009 - minimum regulatory standards were put in place, and schools unable to meet the standards weeded out.
There are now about 300 registered private schools, down from more than 1,000 in 2009.
Of the remaining schools, more are working to attain the four-year EduTrust award - a mark of quality in key areas such as management and providing educational services.
Forty-nine schools had the certification in 2014, up from 34 in 2011.
To earn the award, schools must meet higher standards in areas such as corporate governance, fee protection for students and academic processes like teacher selection.