Scoil Chonglais Baltinglass set up a last-four meeting with Wesley College as they held on to see off a spirited challenge from St Patrick's Classical School Navan in Donnybrook.
The Wicklow school led 7-0 at the break thanks to Shane Sweeney's try from a rolling maul. Jake Elliot brilliantly kicked the conversion from wide on the left.
Elliot extended the lead with a penalty soon after the resumption.
Pat's hit back with a try from Evan Gordon, but they couldn't find another way through a resolute Chonglais defence despite the sin-binning of Jordan Deans.
St Patrick's Navan: R Gordon; S Walsh, O MacGinity, C Morgan, L Morgan; G Brady, K Murphy; L Carroll, M Horton, G Faulkner; Y Gbede, L Ivors; J Condra, B Byrne, P McLoughlin (E Gordon 15).
Scoil Chonglais Baltinglass: A Byrne; J Deans, M Jackson, D Cassels, S Barrett; J Elliot, M Murphy; C Hanley, D Curran, R McCarthy-Griffin; A Bennett, S Brannigan; T O'Toole, S Sweeney, C Jemmaine.
Referee: S Owens