In a Facebook post, Mr Ong said that a decision to reopen all MOE schools after the spring holiday was based on scientific evidence, additional precautionary measures, and a desire to minimise disruption.
Mr Ong cited advice from Professor Dale Fisher, chair of the WHO Global Outbreak Alert and Response Network, and said there is no "evidence to show that the young are vectors or spreaders of the virus".
"In this context, it may not be a bad idea for our children to spend the bulk of their day in school, where lessons and activities are arranged such that they mingle only with their classmates, who are less susceptible to the virus than adults. They will be quite a resilient group.
"If we close schools, many will not stay home, but may run around in the community and mingle with a lot more people, exposing themselves to more risk.
"In that sense, schools remain safe places for children, especially as they seem to be more resilient against the virus."
Mr Ong said that the closure of schools would "disrupt many lives, especially parents who are both working, with no domestic help, and have limited childcare options".
He added: "Keeping our healthcare system strong is paramount in the fight against COVID-19. Our frontline warriors will be much more assured if their children are in school, meaningfully engaged, in a safe and healthy environment."
His response also highlighted extra precautionary measures being taken by the government to help prevent the further spread of Covid-19. These include stopping all short-term visitors from entering Singapore; issuing all students and staff who have returned from overseas on or after March 14 with a 14-day Leave of Absence (LOA); and requiring all students and staff who have returned from overseas on or after March 20 to serve a 14-day Stay Home Notice (SHN).
"We have implemented a Leave of Absence/Stay Home Notice policy, with the result that come Monday, every student, teacher, staff, canteen stall operator in school would not have gone overseas since the start of the March school holidays. As a further precaution, there will be 100% checks on their travel history at the gates."
Read more: What is a Stay-Home Notice?
Schools are also required to implement a raft of new safety measures to protect teachers and students. All co-curricular activities at local schools are cancelled, group sizes in play areas have been reduced; there are more frequent temperature checks in pre-schools; and students are required to sit further apart in all lessons.
Mr Ong added: "Another significant precaution is that every morning, every student who is not feeling well, be it with a cough or sore throat, and not just fever, will have to be in an isolation room or sent home."
The majority of Singapore’s 90-plus international schools are open, or are currently on their spring break. However, last week two of Singapore’s largest international schools decided, unilaterally, to close and move to online learning.
Singapore American School (SAS), which has a student body of 4,000, closed for three days to trial its online learning programme. Tanglin Trust School, which enrols around 2,700 students, closed its campus from last Wednesday until the start of the spring break on March 28.
Read more: Two international schools close temporarily
Some international schools are extending their spring break. For example, International Community School (ICS) has added a second week to its spring holiday and will deliver an online learning programme from March 30 to April 10; the school plans to reopen on April 13.