In 4-quart saucepan, heat sugar, 3 cups of the water and 3/4 cup mint leaves to boiling. Reduce heat; simmer 2 minutes. Cool 1 hour to blend flavors.
Strain and reserve liquid. In nonmetal freezer container, mix reserved liquid, remaining water, orange juice concentrate and lemonade concentrate; freeze until slush consistency.
To serve, spoon about 2/3 cup slush mixture into each serving glass; pour 1/3 cup club soda over each. Garnish with mint leaves.