Friends' School became the first team to qualify for the last-16 of the Danske Bank Ulster Schools' Cup following their 31-17 Group A victory over Rainey in Lisburn.
Nine places remain up for grabs, with the five other group winners and four best second-placed teams going into the draw with the seeded teams - RBAI, Methody, RS Armagh, Campbell College, Wallace High and Ballymena Academy.
Friends', who earned five group points from their first game against Limavady Grammar, claimed another five to ensure their progress.
Winger Fergus McEwen opened the scoring in the first minute with a 50m run-in.
Reece Malone converted and was also on target shortly afterwards when Kyle Gormley broke through the home defence following an offload from captain Jack Harte.
The visitors were then reduced to 14 players and Rainey took advantage when Matthew Brown scored from close range.
After the restart, the trend continued with the visitors' full-back Stuart Patterson crossing out wide before Malone scored from a wrap-around play on the right.
Undaunted, the Magherafelt team deservedly retorted with a Marc Seffen try but a well-worked back move resulted in Malone going in for his second.
The home team gave their all and Friends', who were shown the yellow card twice more in the final 10 minutes, had to withstand some stout Rainey pressure but succumbed to a Daniel Barfoot try right at the end. Matthew Thom converted.
Elsewhere, a last-kick penalty from Harry Caskey gave Coleraine Grammar an 8-7 victory over Belfast Royal Academy.
As both had won their opening game, this was going to be the pivotal clash in Group E and it was a closely-fought affair.
Strong runs by Coleraine captain Luke Bell, Harry Mitchell and evasive full-back Cameron Leighton led to good forward ball but Academy showed good patience and work rate to keep their line intact.
Caskey hit the post with an early penalty attempt but the home side kept up the pressure. Following a scrum on the right, the ball was moved wide and Callum Wallace dashed in. The visitors finished the half strongly but Coleraine led 5-0 at the interval.
The third quarter should have produced more points for the home side but four penalty attempts were missed and when a clear overlap was set up with the line at their mercy, some fine defending by Academy prevented a try.
This failure to score gave renewed confidence to the opposition and with three minutes remaining, Jack Davis crossed for Ollie Parkes to convert.
In injury-time, Coleraine were awarded a penalty from a five-metre scrum and Caskey was successful for a tight victory in front of a jubilant home crowd.
Academy coach David Creighton thought a lack of discipline at the end let his team down but felt there was still all to play for in the final game at home to Banbridge.
Coleraine coach Richard Boyd thought his team showed real grit and spirit. He said: "I feel we were the better side for large parts of the game but failed to take full advantage.
"Academy were well organised and put us under a lot of pressure. We have plenty of areas to focus on and will take confidence into the Belfast High game."
Group A: Friends' 31 (Total points after two of the three games played - 10) Rainey 17 (4), Limavady Grammar 20 (4) Dromore High 19 (1). Group B: Bangor Grammar 47 (10) Down High 19 (5), Regent House 40 (7) Foyle Col 0 (0). Group C: Cambridge Hse 17 (9) Ballyclare High 16 (6), Lurgan Col. 19 (2) Omagh Acad. 19 (2). Group D: Dalriada 40 (5) Strabane Grammar 12 (0), Sullivan Upper 40 (10) Grosvenor Grammar 0 (5). Group E: Banbridge Acad 17 (0) Belfast High 29 (5), Coleraine Grammar 8 (9) Belfast RA 7 (6). Group F: Enniskillen Royal 40 (10) RS Dungannon 10 (5), Portadown Col 38 (5) Larne Grammar 14 (0).