Pupils across Scotland will be required to wear masks in school corridors and communal areas to help prevent the spread of coronavirus.
The recent rule applies from Monday, August 31 and applies to all young people over the age of 12.
Pupils from primary five and above are also to wear face masks on public transport to prevent further transmission of the disease.
The new guidance will be obligatory in all secondary schools, special schools and grant-aided schools, but it will state that pupils should not be excluded from school if they do not wish to wear one.
The change in guidance comes following an update in the advice from the World Health Organisation (WHO) regarding children and face masks.
The announcement to make masks mandatory for secondary school pupils was made by John Swinney, Scotland’s education secretary, after several schools saw a rise in Covid-19 cases in recent weeks.
Last week Kingspark school in Dundee reported an outbreak of coronavirus, with 27 people, including 21 members of staff, two pupils and four community contacts, testing positive for the virus.
About 100 children and staff across the Lothians are also self-isolating after a number of pupils in the region recently tested positive for the virus.
Those schools affected are Granton Primary, Dalry Nursery and Primary, Lasswade High, Balbardie Primary, Preston Tower and Dalkeith High.
NHS Lothian said that, at this time, there is no evidence of transmission within the schools themselves.
Scottish schools reopened on 11 August and the harder line on masks comes after the recent outbreaks seen in among staff and pupils since the school return.
In England, the government made a U-turn last week when it decided that masks should be mandatory in secondary schools - but only in regions where lockdown measures are in place.