The ceremony on Monday comes ahead of controversial double celebrations to mark the impoverished kingdom's 40 years of independence from Britain and Mswati's 40th birthday on September 6.
"Since you know that the country is faced with these major activities, I urge you to behave yourselves and make sure that you display respect so that tourists would return to the country," the Queen mother, Ntombi Twala, told the girls.
The girls were sent to two locations to cut reeds to be used in the annual Reed Dance ceremony at Ludzidzini royal palace near the capital Mbabane.
The age-old reed dance is aimed at encouraging young women to preserve their purity and abstain from sexual activity before marriage.
But the ceremony has seemingly failed to make an impression on the scourge of HIV-AIDS in a country where close to 40 percent of the adult population of one million are infected with the virus, according to a UN report.
The dance spectacle is characterised by bare-breasted maidens, clad in colourful short skirts or sarongs. The king, who has 13 wives, may choose a new bride among the young girls.
The king and eight of his wives returned last week from a controversial Middle East shopping trip in preparation for the so called 40-40 celebrations.
The trip attracted unprecedented condemnation from women's groups, political parties and civil society groups.