Chief Executive Carrie Lam announced on Saturday that Diploma of Secondary Education (DSE) exams are being postponed and civil servants will start working from home again, in light of the worsening coronavirus situation in the territory.
The suggested date for schools to reopen on April 20 has now been dropped, and they will remain closed "until further notice".
The DSE exams will be pushed back by four weeks to April 24. Oral exams for both Chinese and English are cancelled completely.
The postponement affects some 50,000 students in the city.
Meanwhile, government-run sports centres, libraries and museums which recently re-opened are to shut again.
Lam said the situation in Hong Kong is now getting "critical" as many people return to the SAR from areas of the world that have been hard hit by the pandemic.
The CE rejected the idea of a complete ban on non-residents entering the city, saying a series of travel restrictions imposed by other places means the SAR won't receive so many visitors.
She said around 600 non-residents had come into the city on Thursday, and a similar number on Friday, and two-thirds of them were from Taiwan and Macau. These two places are exempt from a recently introduced rule requiring all arrivals to undergo 14 days of self-quarantine.
Lam said there would be "zero tolerance" when it comes to the compulsory quarantine orders, and anyone found to be flouting the rules will be prosecuted immediately.
She said spot checks will be carried out at people's homes, and there will be inspections at places where people are gathering to look for any offenders.
"The 7.5 million Hong Kong people have effectively and safely sailed through two waves of epidemic. Now we are facing a third wave ... we have to adjust and go back to measures that will ensure more social distancing," Lam said.
A large proportion of Hong Kong's 180,000 or so civil servants had first started working from home on January 29, with only those providing essential services required to go into the office as normal.
Many private companies had taken their cue from the government, telling their staff not to come in if they didn't need to, or staggering shifts to reduce to contact between employees.
But around five weeks later, the government's work-from-home arrangement came to an end as Hong Kong recorded relatively few cases of the new coronavirus, and the streets and public transport started filling up again.
In recent days, however, the number of infections has soared, hitting 273 by Saturday evening, with four fatalities in the SAR since the outbreak began.