Brynteg scored first through flanker Jack Walker, centre Ieuan Thomas adding a conversion but Cowbridge came straight back with a try and conversion for outside half Bevan Howells. Brynteg prop Chris Crumpton claimed his side's second but Cowbridge were bigger physically which made the difference in the end.
They scored three unanswered tries through centre Rhydian Williams, hooker Max Lucas and openside flanker Sam Corkhill, Bevan Howells adding two conversions.
The Dewar Shield was a tight affair, neither side giving an inch. In a combative, passionate encounter, Rhondda just had enough to squeeze in for two tries from close range scrums through scrum half Callum Phillips (Ysgol y Cymer) and Matthew Davies (Ferndale), Grant Smith adding a conversion.
Tomorrow, Whitchurch High and Ebbw Fawr will contest the Welsh Schools Under 16 Cup Final at 2.30pm, again at the Millennium Stadium.
Entry is free for this match and the game will also be streamed live on s4c.co.uk and extended highlights will be shown on Rygbi Pawb at 22.30.