Pupils and teachers have been asked to self isolate with schools across Wales affected by positive Covid-19 cases.
Areas affected include Bridgend, Cardiff, Caerphilly, Carmarthen, Gwynedd, Neath, Rhondda, and Wrexham.
More than 200 pupils at Bryntirion comprehensive in Bridgend have been asked to isolate after a confirmed case.
And 30 pupils in Year 7 class at Ysgol Bro Edern, Llanedeyrn, Cardiff, must self-isolate for 14 days after a case.
At Bryntirion, all Year 7 pupils have been asked to stay away as they have been identified as having had potential contact with a person who has tested positive, along with three staff members.
All other year groups have been advised to continue to attend school.
Head teacher Ravi Pawar said: "As soon as the school was informed about the positive test result, prompt action was taken to safeguard the well-being of pupils, teachers and staff.
"Their continued well-being remains a top priority, and the school is continuing to work closely alongside the council and Public Health Wales to ensure that lessons can continue as normal.
"We are using online access, virtual classrooms and blended learning techniques to ensure that all those affected can continue to participate in and benefit from lessons."
Elsewhere in Bridgend, staff members at Maesteg Comprehensive and Ysgol Bryn Castell have tested positive and advised to isolate for 14 days.
A class of 21 pupils at a primary school in Bargoed, Caerphilly county, were told to self-isolate for two weeks after a member of staff tested positive.
In Torfaen, staff at Woodlands Community Primary in Cwmbran are self isolating after a staff member tested positive.
Neath Port Talbot council said there have been positive tests on a pupil and a staff member at Cwmnedd and Coedffranc primary schools.
Both schools remain open with eight pupils and three staff self-isolating at Cwmnedd primary, and four staff at Coedffranc.
In Wrexham, St Christopher's special school for pupils aged six-19 sent pupils home after a teacher tested positive.
It should have been pupils' first day back, but it will now try to reopen on Wednesday following the "isolated incident".
A staff member at Ysgol y Dderwen Primary in Carmarthen has also tested positive.
The county council said it was being dealt with "in line with its test, trace and protect procedures".
In Rhondda Cynon Taff, some pupils and staff are self-isolating at Pontygwaith and Cymmer primary schools, according to the council.
It said: "All schools with confirmed cases are deep cleaned to minimise risks in affected areas and strict preventative controls will continue to be operated."
In Gwynedd, Ysgol Friars in Bangor has closed its kitchen for deep cleaning after a worker tested positive. Children have been asked to bring in a packed lunch as the school remains open.
Sion Amlyn from teaching union NASUWT Cymru said teachers at affected schools could expect to carry out "blended learning" with pupils at home and others in classrooms.
"The expectation is that there will be a continuation of a mix of face-to-face teaching, and remote teaching going ahead into the future," he told BBC Radio Wales' Gareth Lewis programme.
Dr Giri Shankar, in charge of Public Health Wales' response to the pandemic, said parents would be concerned about positive cases in schools.
"I would urge you to follow advice from the school or local authority and only withdraw children from school when asked to do so," he said.
"Social distancing measures that are in place are designed to prevent the spread of the disease beyond the 'bubbles' established in schools.
"Again it is important that children maintain regular handwashing throughout the day, and on returning home, and parents' help with this is vital."