Limavady Grammar were only one of two teams to win away from home in the second round of the Danske Bank Schools' Cup.
They forged into a 20-8 interval lead and although Portadown narrowed the gap to three points at one stage the visitors ran in a late try to seal the win.
Limavady out-half Scott McLean controlled the game with some outstanding play. He is only one of two players back after Limavady reached the semi-finals of the Subsidiary Shield last season.
Niall Davidson kicked an early penalty for Portadown, but Limavady levelled with one from McLean. They went in front when Oliver Mullan dived over from a driving maul for McLean to add the points.
Scott Patton touching down with McLean again on target with the extras and he then kicked a penalty to make it 20-3.
Jordan Dowds scored to bring Portadown back into the encounter and a Jacob McKeeman try then made it interesting.
Limavady's Jack Connor crossed after a pick and go before one of the home replacements Stewart Hodgen made his presence felt with a converted try.
That brought Portadown within striking distance, but Mark Woods made sure of the win for Limavady.
"We were more clinical and our driving mauls were paying dividends," said Limavady coach Chris Nash.