The boys, some as young as 13, were abducted near Malakal, the capital of Upper Nile state, according to a UNICEF statement.
Malakal, which is currently under government control, has recently seen intense fighting between rebel forces and government troops who accuse each other of violating multiple peace agreements.
Armed men surrounded the community and searched house by house, forcibly taking boys older than 12, said witnesses, according to the UNICEF statement, which said the latest abductions were carried out while the children were doing their exams in the Wau Shilluk area. The number of those abducted could be much higher, it added.
Last year 12,000 children were used as soldiers by armed forces and groups across South Sudan, according UNICEF.
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"The recruitment and use of children by armed forces destroys families and communities. Children are exposed to incomprehensible levels of violence, they lose their families and their chance to go to school," said Jonathan Veitch, the UNICEF representative in South Sudan.
Watchdog groups have persistently accused South Sudan's warring factions of actively recruiting and using child soldiers. South Sudan's military and rebels are actively enlisting children despite promises to the contrary, Human Rights Watch said in a report last week.
South Sudan's current conflict started in December 2013 when forces led by former Vice President Riek Machar fought troops loyal to President Salva Kiir.