David White kicked a late conversion to clinch a Sussex Schools' under-18s Cup hat-trick for The Weald.
The Billingshurst school added to their 1996 and 1997 triumphs by pipping favourites Sussex Downs College (Lewes) 20-19 on finals day at Worthing.
White's touchline effort came after a Sam Rickman try tied the scores with eight minutes left.
Mike Webb, head of physical education at The Weald, said: "It took a lot of nerve on David's part. It was a difficult kick. It was excellent to secure our third win, particularly as we were the underdogs against a bigger side representing a sports college."
In a tight tussle, The Weald moved the ball out wide as often as possible. Chris Tidman touched down twice and White also kicked a penalty.
Sussex Downs were hoping to win the competition for Steve Richardson, their hooker who died from cancer 12 days ago.
Players wore black armbands and there was a minute's silence in his memory before the kick off.
Coach Leigh Puttock said: "There is no question The Weald deserved to win it.
"My boys wanted to win it for Steve. They were very emotional. They were tense and didn't play as well as usual.
"Tom Saunders and Rob Jackson did well but the boys voted substitute Ollie Horscroft their player-of-the-match after he came on for the injured Tom Burton at prop."
The Weald: Andrew Broughton, Sam Rickman, Andrew Pull, Matthew Goff, Christopher Tidman, David White, Rupert Davenport, Tom Ruff, Gareth Beal, Daniel Ingram, Anthony Gould, Aubrey Plumer, John Francis, Nathan Fleming, Daniel Harding, Matthew Wilson, Tom Filby, Peter Howard-Smith, John Box, Joe Twelvetrees, Peter Phillipps .
Sussex Downs College: Adam Makepeace, Stephen Dale, Tom Burton, Nick Sharp, Dean Ross-Jones, Djamal Sedour, Adam Lawrence, Brendan Crean, Matt Walker, Simon Lewis, Tom Saunders, Brett David, Kyle Lusk, David Murray, Rob Jackson, Ollie Horscroft, Dan Cartlidge, James Newman, Tom Evans, Stephen Richards.
Chris Eubank was a knockout as Brighton College won the under-13s crown.
The winger, name after his father, the former world boxing champion, scored four tries as Brighton beat Hove Park 29-15.
Coach Tim Herbert said: "Chris is a tremendous athlete, very fast. He was second in the 100m in the independent schools' national athletics finals. He showed speed with his counter-attacking tries. He is well built and powerful, which is not surprising considering his father's sporting background.
"His dad is supportive but he keeps a low profile on the sports side so his kids can achieve on their own merit.
"His younger son, Sebastian, helped us reach the semi-finals of the under-12s."
Captain Ryan Bhimji also touched down and Chris Cocum kicked two conversions. Herbert said: "Ryan was one of our star players. Credit to Hove Park, they had more possession but we defended superbly."
Hove Park replied through Luke Thompson, who scored a penalty and a conversion following tries from Adam Savage and Josh Avis.
Hurstpierpoint College defeated Longhill High 26-10 in the under-15s final with tries by Noel Cato (2), Toby Sutton and Dan Barton, plus three conversions from Kelvin Gatungo.
Longhill, who were runners-up last year, put up a fierce resistance and Sussex player Will Smith scored a try converted by Harry Coker who also kicked a penalty.
Chichester High overpowered spirited St Bedes (Upper Dicker) 24-12 in the under-16s final.
Kenny Alegbe (2), Adam Shaw and David Denton touched down, Denton added a conversion.
The under-12s cup was shared.
Imberhorne (East Grinstead), national finalists, were held to 14-14 draw by a Warden Park (Cuckfield) side coached by Haywards Heath 1st XV coach Steve Heaton.
Warden Park fly-half Ross Chisholm scored and converted two tries, while Chris Presland and Liam Andrews responded with tries. Presland kicked two conversions.
Eastbourne College and Oathall (Haywards Heath) shared the under-14s' trophy after a 10-10 draw.
Oathall scored tries through Gilly Burns-McClain and Greg Jackson and Andrew McClaren and Lewis Munro replied.
Trophies were presented by Lucy Reimoser of sponsors Lloyds TSB Registrars.
The Cup competitions involved 150 teams from 50 schools.
BRENDAN Crean (Sussex Downs, Lewes) has been called up by England.
The second-row prop, 17, has been selected for the English Colleges squad for two matches against Wales and one versus Scotland.
Sussex Downs rugby coach Leigh Puttock said: "Brendan is very athletic, has a good rugby brain and is a hard worker.
"He was tipped for England Schools' this year but just missed out. We are delighted for him.
"We had former student Marcus Main called up for England Students' last season, but Brendan is the first to be chosen for his country while still at the college.
"Brendan is also a highly respected member of the Harlequins under-19s team."
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