Same milieu, different solution to starting the F-word:
Which reminds me, as a high school upperclassman who didn't use the F-word, I did say 'shit' pretty often. Which was a potential problem for me when playing tennis -- plenty of mistakes occurred that inspired the exclamation, but swearing was bad form and could get me and my doubles partner in trouble. So I started substituting. Since I'd get the 'shhhh' sound out pretty routinely, I went with a 'nonsense word' that matched: the French word for 'smurf', which is 'schtroumpf'. This seemed appropriate to me, since as any child of the 80s could tell you -- even one like me who was only allowed to watch public television -- 'smurf' can mean anything!