Fletcher's side had taken a fourth minute lead through Newcastle Falcons speedster Zach Kibirige, converted by Northampton Saints fly half Sam Olver. But France hit back just five minutes later, scoring a penalty and a well taken pick-and-go try which was also converted.
Two Olver penalties and one more from France made the score 13-13 before the men in blue scored the decisive try in the second half.
Fletcher said: "Our work ethic was very good today but at times the backs lacked consistency and made some unforced errors. Our forwards were excellent all game though and we really came into it in the last 20 minutes, the lads gave their all, it just wasn't to be.
"The French defended well - they're a well organised side and to be fair they probably just edged the game, but it was very close."
Forwards Coach Pete Walton added: "The forwards put a real shift in today. France are a big strong, physical team but we scrummaged very well. That was a full strength French side but we matched them, Biyi Alo in particular was outstanding, but all of the front row played their part."
England now prepare to face South Africa on Cape Town on Saturday.
England points: Zach Kibirige 1T; Sam Olver 1C, 2P.
England Under 18 starting XV against France
15 Howard Packman (Moulton College) (Northampton Saints)
14 Zach Kibirige (Yarm School) (Newcastle Falcons)
13 Nick Tompkins (Oaklands College) (Saracens)
12 Tom Stephenson (Moulton College) (Northampton Saints) (C)
11 Joshua Watkins (Bromsgrove School) (Worcester Warriors)
10 Sam Olver (Oundle School) (Northampton Saints)
9 Ollie Claxton (Adams Grammar School) (Unattached)
1 Alex Lundberg (Hampton) (London Wasps)
2 Sam James (Bishops Stortford) (Saracens)
3 Biyi Alo (Haileybury College) (Saracens)
4 Jake Oldaker (Wellington College) (Bath Rugby)
5 David Hughes (Twyford C&E High School) (London Wasps)
6 Maro Itoje (Harrow School) (Saracens)
7 Steve Uren (Filton College) (Bristol Rugby)
8 Joel Conlon (Ivybridge Community College) (Exeter Chiefs)
16 Gus Jones (Eton College) (London Wasps)
17 Harry Thacker (Leicester Grammar) (Leicester Tigers)
18 Charlie Ewels (Bryanston School) (Bath Rugby)
19 Danny Hobbs-Awoyemi (Moulton College) (Northampton Saints)
20 Callum Braley (Hartpury College) (Bristol Rugby)
21 Nick Carpenter (Filton College) (Bristol Rugby)
22 Billy Burns (Hartpury College) (Gloucester Rugby)