Building work is going on at the school after a 2006 arson attack
Part of a Swansea comprehensive school could be closed for up to a fortnight after asbestos was found by contractors installing new windows.
The asbestos which is now being made safe by specialists was found in the annex of Penyrheol school at Gorseinon.
The school said there was no risk to students or staff because the absestos had not been disturbed when found.
But they will not be able to return to the 16 annex classrooms until the council is happy safety is guaranteed.
The school will only be open on Monday to staff following the Easter holidays to enable teachers to reorganise teaching schedules for the coming fortnight.
The other 35 classrooms will not be affected by the works and will reopen on Tuesday.
However, the annex closure means only three year groups can use the school each day at a time.
Priority will be given to GCSE pupils in years 10 and 11 and they will be taught as normal from Tuesday.
But the other year groups will be taught on a rotational basis on different weekdays and parents will need to contact the school directly from Monday to find out when their children are being asked to attend.
Head teacher Alan Toothill said: "This is an unfortunate situation but expert works are now going on to deal with the issue.
"The annex will be be closed off during the works and I'd like to reassure parents that there's no risk to pupils.
Coun Mike Day, Swansea's Cabinet Member for Education, said: "The priority here is the health and safety of pupils and staff at Penyrheol Comprehensive so this is why the annex will be closed while the remedial works take place.
A new school building is currently under construction at the Penyrheol site after an arson attack gutted the teaching block in 2006.
Parents of years seven, eight and nine pupils can contact the school from Monday for class times on 01792 533066 or via the internet at www.penyrheol-comp.net