The school is likely to remain closed for 10 days

One of the three was arrested on Saturday night and the other two early on Sunday, say South Wales Police.

Officers still want to trace a group of youths seen at Penyrheol Comprehensive in Gorseinon just before the blaze.

Education officials and senior staff met at the school site but pupils will not be back for at least a week.

About 40 classrooms in the main block were destroyed but a leisure centre and annexe on the same site escaped with smoke damage.

Pupils who are coming up to sitting their examinations in the not too distant future... have course work which has gone up in flames here Head Alan Tootill

Supt Julian Williams, from Swansea Central Police Station, said: "We are asking that any people with information about the fire, especially the group of youths who were in this area at the time, to contact ourselves so that we can progress the enquiry as quickly as possible.

"The fire is being treated as suspicious at the moment with a joint investigation taking place between ourselves, the fire service and the forensic science service."

The fire broke out at about 0100 GMT on Saturday, gutting the 1960s three-storey main building and causing millions of pounds of damage.

Firefighters called in from across mid and west Wales tackled the blaze for four hours and managed to stop it spreading to a nearby leisure centre and theatre, using water from the swimming pull to douse the flames.

Only the English and maths departments housed in an annexe were undamaged.

Head Alan Tootill said: "My thoughts initially were with the pupils and their parents, particularly the pupils who are coming up to sitting their examinations in the not too distant future who'll have course work which has gone up in flames here.

The school will remain closed for at least a week

"And my thoughts are also with the staff, many of whom have worked here for a long time and in many ways their life's resources, their notes, were also destroyed."

Education officials said the school would be closed for up to 10 days while alternative arrangements are made.

Temporary classrooms

Senior staff and Swansea council officials meet at the site on Sunday where fire crews are continuing to damp down to make an initial assessment of the extent of the damage and look at how best to accommodate the pupils.

The preferred option is to remain on the site, using a combination of buildings which escaped the blaze and temporary classrooms.

Mike Day, Swansea Council cabinet member for education, said everyone at the school had been devastated.

They were trying to minimise the disruption for pupils, but parents would need to be patient, said Mr Day.

Only English and maths areas survived the inferno unscathed

"Of course, at this time of year what we're concerned about are those pupils who will be doing their GCSEs."

The school had just been refurbished and the swimming pool reopened only last week.

The leisure centre which suffered smoke damage is to remain closed until further notice and school outdoor pursuit trips to Gower planned for this week have been cancelled.

Parents have been advised by the council to make alternative arrangements for looking after their children while a way forward is found

They should also check the council's web site and media for any updates.

Anyone with information can call police on 01792 456999 or Crimestoppers on 0800 555111.