2 cups water
1/2 cup white sugar
2 black tea bags, or more to taste
1 cucumber, cut into 1/4-inch slices
5 cups sparkling water, or as needed
Bring water to a boil in a kettle. Pour into a pitcher. Stir in sugar until it dissolves into a syrup. Add tea bags; let steep, about 5 minutes.
Fill 5 glasses with ice cubes. Add some cucumber slices. Fill glasses 3/4 full of sparking water. Divide sweet tea evenly among glasses. Squeeze some lemon juice into each drink; stir.