In a very large heatproof measuring cup, combine 8 cups boiling water with the tea bags and dried lavender flowers; let steep for 10 minutes. Strain the tea and let cool. In a heatproof bowl, combine the sugar with the remaining 1/4 cup boiling water and stir until the sugar dissolves. Add the sugar syrup to the tea and let cool. Serve the tea in tall ice-filled glasses, garnished with lavender or mint sprigs.