Combine sugar and juice in a small saucepan; bring to a boil. Reduce heat and simmer 1 minute, stirring until sugar dissolves. Remove from heat. Stir in the zest. Cover and refrigerate overnight. Strain mixture through a fine sieve into a bowl; discard solids.
Combine lemon syrup and prosecco in a pitcher. Pour about 1/2 cup prosecco mixture into each of 12 Champagne flutes; garnish each serving with 3 raspberries. Serve immediately.