The fire on 1 January destroyed the nursery and infant classrooms at Coed Eva Primary School, Cwmbran.
The teenager, together with six others, was arrested on suspicion of arson with intent to endanger life.
The others, two aged 15, two aged 19, a 16-year-old and a 21-year-old, were released without charge last month.
A reward of up to ??5,000 is being offered by Crimestoppers for information about the fire that leads directly to an arrest and conviction.

Welsh activist John Rees-Evans said Mr Nuttall was effectively powerless without the support of the national executive committee (NEC).
Elections are currently taking place for the ruling body.
Former leader Nigel Farage had a difficult relationship with the NEC.
Mr Rees-Evans came third in the leadership contest with 2,775 votes (18.1%). Mr Nuttall won with 9,622 votes (62.6%), while Suzanne Evans came second with 2,973 votes (19.3%).
"Constitutionally, the role of the leader is limited in terms of determining the direction the party may take," Mr Rees-Evans told BBC Radio Wales' Good Morning Wales programme.
"He needs the support of the NEC and if he doesn't have that support he is effectively powerless."
He hoped that Mr Nuttall would get that support.
"Right now we're in the middle of an NEC election," he said.
"I shall do my very best to try to support candidates that I think will back him and do believe in some of the principles I have stood for very overtly during my campaign.
"I'm hopeful he will get it, but under the present NEC I'm afraid I wouldn't be hopeful."
The NEC was alleged to have "defied" former UKIP Wales leader Nathan Gill over the selection of assembly candidates at the beginning of 2016.
Ex-leader Nigel Farage had publicly criticised one UKIP assembly candidate, Gareth Bennett, as not the sort of person the party would be proud to have as an assembly member.
But despite that the NEC rejected calls to deselect him. Mr Bennett went on to become an AM.
A 15-year-old boy who was arrested following an arson attack on a Torfaen school has been released without charge.
[NEXT_CONCEPT]
A defeated UKIP leadership candidate has said he is not "hopeful" that new leader Paul Nuttall will get the support he needs from the party's ruling body.