Concerns are mounting after Covid-19 cases were detected among students when schools resumed on Monday.
Initially a false alarm was suspected when antigen tests kits returned positive results at Kham Soi Phitthayasan School in Mukdahan's Nikhom Kham Soi district, but further RT-PCR tests all proved negative.
Opas Karnkawinpong, director-general of the Department of Disease Control, said that 14 teachers and three students initially tested positive using ATKs procured by the school on Thursday.
Provincial public health officials then carried out rapid antigen tests on a further 1,106 students and school personnel and 87 of the results came back positive.
However, Tharit Prasarnsorn, the Education Ministry's inspector-general, said that all the tests had since been proven to be false positives.
Nevertheless, the school has been ordered closed for 10 days with classes to continue online, Mr Tharit said.
Government spokesman Thanakorn Wangboonkongchana said on Friday Prime Minister Prayut Chan-o-cha is concerned about the possibility of infections detected among students and teachers and instructed relevant agencies to take all due care to prevent any outbreaks.
The prime minister also praised officials for their quick response to the situation in Mukdahan, Mr Thanakorn said, adding that more than 70% of the students and teachers at the school in Mukdahan had been vaccinated.
The spokesman gave assurances that the government weighed the risks carefully before deciding to allow schools to reopen this month, and on Nov 15, schools under the supervision of the Bangkok Metropolitan Administration and other agencies will also reopen, Mr Thanakorn said.
Amporn Pinasa, director-general of the Office of the Basic Education Commission (Obec), said that on-site learning resumed at about 12,000 schools nationwide on Nov 1.
He said he has been informed of a teacher at a school in Bueng Kan testing positive after taking part in a volleyball contest elsewhere.
The teacher has been quarantined and the school has been ordered closed for seven days for disinfection, Mr Amporn said.
In Phichit, Aphisit Wisit, a deputy provincial public health officer, said that two 9-year-old students at Ban Sa school in Wachira Banrami district were found to be infected.
They got the virus from a relative who came from Bangkok. After returning to the capital, the relative phoned to tell the boys' family that he had contracted the virus.
The boys then were tested and found to be carrying the virus, Mr Aphisit said.
The boys had not been vaccinated as they are still too young, according to current rules, Mr Aphisit said, adding that the boys went to school on Nov 1 and the school has been ordered closed temporarily.
Health officials then carried out tests on high-risk people including 16 students and 89 local residents who may have come into contact with the boys.
Of them, 11 tested positive and the results are pending for the rest, Mr Aphisit said.
In Lampang, Prasert Kitsuwannarat, the provincial public health officer, said that ten students at the Thoen Technological College had been diagnosed with Covid-19.
It is believed that one of the students contracted the virus in Tak and then spread it to nine of the 13 other students with whom he shared a dormitory, Dr Prasert said.
In Ubon Ratchathani, Pathum Phitthayakhom School in Muang district were ordered closed for ten days, only three days after reopening when a student contracted Covid-19.
The student was infected by a friend from another school, its director Jirayuth Aksornphim said.