Combine the sugar and water in a small saucepan and simmer until the sugar dissolves. Remove from the heat and add the mint leaves. Allow to steep until cooled. Once cooled, strain out the leaves.
Combine half of the mint syrup, 4 cups ice, and 1 cup bourbon in a blender. Blend until smooth. Pour into a pitcher. Repeat with the remaining ingredients. Rub the rims of julep cups, if available, or wide-mouthed, long-stemmed glasses with orange slices and dip the rims in sugar, then fill with the frozen mint juleps. Garnish each with an orange slice and mint sprig and serve immediately.