The Leicester Tigers centre, named on the wing in the national team, made a dream start, scoring with almost his first touch as England produced one of their best attacking passages of the championship three minutes in.
They forced a turnover, then kept their shape through several phases before Joe Marler stormed up the middle with Jamie George supporting, before Rory Clegg slid a low kick between defenders for Forsyth to touch down under the posts.
Marcus Watson also scored on his debut as England got their Under-20 RBS 6 Nations campaign back on the rails and lock Charlie Matthews (Harlequins) added a third try just before the interval.
Discipline, though, was an issue as England lost three players to the sin bin – Leicester Tigers lock Calum Green for fighting, Sam Smith of Harlequins for a tackle on an airborne defender and Newcastle flanker Will Welch for killing the ball inside his own 22.
Forsyth, from Wakefield and with previous caps at Under-18 level , said: "I went to Portugal for Under-20s December training camp and didn't make it into the 32 man squad. But I kept in touch and [head coach] Mark Mapletoft said 'You have to show that you're capable of and if you keep playing well then you'll get the call-up'.
“Luckily there have been a few injuries and I've got a chance.
"Now I'll try to just keep doing what I'm doing, play every game as well as I can and hopefully get on the plane to Argentina this summer."
Forsyth even picked up the man of the match award, even if it was a case of mistake identity as his name - rather than that of Casson - was announced in error at the Glasgow Warriors ground.
Scotland U20: Tom Brown (Edinburgh Ac); Oliver Grove (Worcester Warriors), 13 James Johnstone (Currie), 12 Alex Dunbar (Selkirk), Dougie Fife (Currie); Alex Blair (Edinburgh Ac), Alex Black (Leeds Carnegie); Nicky Little (Hawick), Alun Walker (Currie), Colin Phillips (Stewart's Melville), Matthew Reid (Heriot's), Robert Harley (West of Scotland), Michael Maltman (Heriot's), David Denton (Edinburgh Accies), Stuart McInally (Watsonians, captain).
Replacements: Lindsey Gibson (Melrose) for Walker 72, George Hunter (Glasgow Hawks) for Phillips 58, Aaron Hall (Newcastle Falcons) for Reid 48, Callum Stidston-Nott (London Scottish) for Maltman, Kris Hamilton (Caithness), Matthew Scott (Currie) for Johnstone 53, Duncan Weir (Glasgow Hawks) for Grove.
Scorers: Penalties: Blair 2.
England U20: Tom Catterick (Newcastle Falcons); Marcus Watson (London Irish), Andrew Forsyth (Leicester Tigers), Tom Casson (Harlequins), Sam Smith (Harlequins); Rory Clegg (Harlequins), Sam Harrison (Leicester Tigers); Lee Imiolek (Sale Sharks), Jamie George (Saracens), Joe Marler (Harlequins), Calum Green (Leicester Tigers), Charlie Matthews (Harlequins), Jamie Gibson (London Irish), Jacob Rowan (Leeds Carnegie, captain), Jackson Wray (Saracens).
Replacements: Arthur Ellis (London Wasps) for George 63, Mako Vunipola (Bristol Rugby) for Imiolek 53, Danny Wright (Gloucester Rugby) for Green 53, Will Welch (Newcastle Falcons) for Wray 56, Jordi Pasqualin (Gloucester Rugby) for Harrison 76, Mark Atkinson (Sale Sharks) for Casson 61, Jonathan Joseph (London Irish) for Watson 69.
Scorers: Tries: Forsyth, Watson, Matthews. Conversions: Clegg 3. Penalties: Clegg 2.