Crickhowell High School approached Powys council offering to manage the town's library to benefit pupils and the community.
It would mean the service can meet its local savings target.
The library would stay in its Silver Street location, and the council said the handover would be seamless with no disruption to the public.

The planned opening of the primary and secondary schools, in the Castel, has already been delayed by a year and is now scheduled for September 2018.
Chief Minister Jonathan Le Tocq said putting the decision back by a month would not cause further delays.
He said the time would be used to ensure points raised had been answered.
Deputy Le Tocq said the Policy Council had only had the chance to discuss three elements - the Education Department's report, the Treasury and Resources Department's comment and the independent review panel's report - on Monday.
He said: "Having discussed all three, I will be actively encouraging both the Education and the Treasury and Resources Departments to address points raised, in order to ensure we demonstrate a balance between the needs of our community's education system and best value-for-money for taxpayers."
The plans include an autism centre and improved sports facilities.
The secondary school is the last of the States high schools to be redeveloped, following the rebuild of Les Beaucamps and St Sampson's.

The US administration signalled heavy cuts to UN operations, in its budget proposals released on Tuesday.
The US foots more than a quarter of the UN's $7.9bn (Â£6.1bn) peacekeeping bill.
A spokesman for UN Secretary General Antonio Guterres said the organisation was willing to discuss how peacekeeping could be made more cost-effective.
Mr Trump's budget proposal declares new "attention on the appropriate US share of international spending at the United Nations".
The document does not provide a detailed breakdown, but Reuters news agency reports the drop in funding for the operations could amount to $1bn.
The spokesman said the UN was studying Mr Trump's plan. "The figures presented would simply make it impossible for the UN to continue all of its essential work advancing peace, development, human rights and humanitarian assistance," he said.
The BBC's Nick Bryant at the UN says such a warning from the organisation is unusually blunt.
The US president does not set the budget, but makes recommendations to Congress. It is understood that the UN secretary general has been lobbying Congress members on the importance of international peacekeeping.
The US provides 28.5% of the United Nations peacekeeping budget - almost three times as much as the next-highest contributor, China.
While Mr Trump's plan does not include details, the US is known to want to cap its contributions to a maximum of 25%.
There are currently 16 active peacekeeping operations, including Syria, South Sudan, and on the border of India and Pakistan.

Police found Kevin Malthouse in Blackshots Lane, Grays, in the early hours of Saturday morning following reports of a group of men fighting.
The 24-year-old had been stabbed in the chest and died at the scene.
Jack Taylor, 18, from Oakley Close, Grays, has been charged with his murder and is due to appear at Basildon Magistrates' Court.
Flowers - including moving tributes from Mr Malthouse's parents and wife - have been laid near the spot where he died.
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A 22-year-old man, who was also arrested, has been bailed pending further investigations.
A 21-year-old man, from South Ockendon, was released without charge and faces no further action.
Mr Malthouse's former team Tilbury Football Club has paid tribute, saying he was "a lovely lad".
A Powys high school has offered to take over a town library to secure its future.
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A final decision on the £59m redevelopment of La Mare de Carteret schools will not now be made until May, Guernsey's Policy Council has ruled.
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The United Nations has warned that President Donald Trump's plans to cut contributions to peacekeeping will make such work "impossible".
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A man has been charged with murder after a former non-league footballer was stabbed to death.