Opec secretary-general Attapol Truektrong said around 100 private schools in Bangkok, Samut Sakhon and several nearby provinces have closed because of the increasing number of confirmed cases linked to a seafood market in Samut Sakhon.
Private schools are allowed to close or remain open at their discretion, the secretary-general said.
A number of other schools have begun allowing at-risk students and staff to work from home from Dec 21 to Jan 3.
Mr Attapol said he had not been informed of Khlong Toei Wittaya School's closure due to a confirmed case in the school's neighbourhood.
"The school could close immediately if it finds itself at risk of an [Covid-19] outbreak," he said.
That said, Mr Attapol said that the closures are unlikely to significantly affect students as it is now the long holiday season.
Education Minister Nataphol Teepsuwan said the ministry had already prepared for a new wave of infections and reassured parents that all schools are capable of teaching their students during the pandemic.
The minister instructed schools in Samut Sakhon to provide online or DLTV satellite learning from Dec 21.
Some schools under the supervision of the Office of the Basic Education Commission (Obec) whose teachers live in high-risk areas have been advised to switch to online learning, Mr Nataphol said.
Obec's secretary-general, Amporn Pinasa, said he had ordered all schools under the supervision of Obec in Samut Sakhon to shut. Some schools in nearby areas have also been closed, bringing the total number of closed schools to 129. Schools with a high risk of exposure to the virus are allowed to close immediately, he said.
Samut Sakhon has been put under a lockdown, with a curfew imposed from 10pm-5am until Jan 3, after confirmed Covid-19 cases there surged by more than 500 in one day. Most of the infected are migrant workers from Myanmar.
A fresh market in tambon Mahachai of Samut Sakhon's Muang district is now the hotspot of a Covid-19 outbreak which has spread to several other provinces, including Bangkok.
The 360 local infections in Samut Sakhon were among 382 new coronavirus cases reported on Monday, raising the nationwide total to 5,289, with no new deaths to date.