Aquinas Grammar in Belfast took the top spot at the Mock Trial contest, beating off stiff competition from 16 other schools across the UK.
The school travelled to Cardiff on Saturday to compete in the 23 year-old tradition, which sees students perform as jurors, barristers and witnesses in a court room scenario.
Dalriada School in Ballymoney also made it to the finals in Wales.
The schools performed in front of judges including Mark Mulholland QC, Chairman of the Bar Council Northern Ireland.
The competition is run by the Citizenship Foundation and funded by the Bar, through the Bar Council, the Faculty of Advocates, the Bar Library of Northern Ireland, the Inns of Court and the Circuits.