The jocks at McKinley High better watch their backs --- because Kurt can kick serious butt!

'Glee' stars Matthew Morrison, Chris Colfer and Amber Riley were all guests on the BBC's "Friday Night with Jonathan Ross" -- where Chris showed off some killer tricks with a pair of sai swords!

Morrison and Riley took cover behind the couch, just in case Chris fumbled -- but kid proved to be a real pro and didn't drop either weapon once.

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