England went down 21-9 to their Welsh counterparts, though it was full-back Tonks who went closest to scoring a try, only to be denied by a high tackle which was punished by a penalty kick from Newcastle fly-half Rob Miller.
Tonks had got within metres of touching down after a line-out. Scrum-half Dave Lewis had recycled the ball out to the midfield and Tonks pounced on it, only to be brought down just short of the line.
England head coach Chris Kibble said: "We got ourselves into the right positions and we had a very good spell in the second half but we paid the price for playing a bit naively. Often we fired extra passes when we needed to hang on to the ball. We also forced too many errors. It is a bitter pill to swallow, but we will pick ourselves up ahead of our next game against Ireland on Saturday."
England Under 18s Six Nations Festival team
15 Greig Tonks (Nottingham High School & Leicester Tigers), 14 Sebastian Stegmann (Cranleigh School & NEC Harlequins), 13 Ben Williams (Bryanston School & Bath Rugby), 12 Mike Stanley (Oaklands College & Saracens), 11 Charlie Sharples (St Peters High & Gloucester), 10 Rob Miller (Keswick School & Newcastle Falcons), 9 David Lewis (Ivybridge Community College & SWERA); 1 Ben Moon (Ivybridge Community College & SWERA), 2 Tom Collett (Wyggeston QEI College & Leicester Tigers), 3 James Currie (Millfield & Bath Rugby), 4 Graham Kitchener (Adams Grammar & Worcester Warriors), 5 Dan Williams (Ivybridge Community College & SWERA), 6 Calum Clark (Barnard Castle & Leeds Tykes), 7 Jacob Rowan (Bradford Grammar & Leeds Tykes), 8 Carl Fearns (Sedbergh & Sale) (captain).
Replacements: 16 Joseph Marler (Sussex Downs & NEC Harlequins), 17 James Clark (London Irish), 18 Alex Brown (Dulwich College & Saracens), 19 Courtney Lawes (Moulton College & Northampton Saints), 20 Oliver Frost (Bromsgrove School & Worcester), 21 Jack Cobden (Wyggeston QEI College & Leicester Tigers), 22 Tom Sargeant (Royal Grammar Guildford & NEC Harlequins), 23 Luke Eves (Colston's Collegiate & Bristol Rugby), 24 Mike Macfarlane (Royal Grammar Guildford & NEC Harlequins), 25 Mark Davies (St Peters High & Gloucester Rugby), 26 Henry Trinder (Hartpury College & Gloucester Rugby).
Substitutions: Sargeant for Rowan (26) (blood), Rowan on for Sargeant (51), Marler for Moon (53), Frost on for Lewis (56), Brown on for Currie (57), Lawes on for Kitchener (70).
