Is there not a dress code guide posted on the doors of the chapel? There was at the chapel I went to.
The problem is that many people would dress modestly
only for Sunday Mass. Then, you see the same people during the week dressed as the rest of the secular world (basically in grossly immodest rags). They are practically indistinguishable. Unless the parents insist heavily on keeping the Traditional Catholic modesty standards at home, this seems to be the norm.
Sermons and dress codes do help, of course; but unless the modesty is practiced and reinforced at
home,
and since infancy, they will not be sufficient. There needs to be a deep contrast between the standards of the Catholic home, and the worldly fashions, especially nowadays, when society has lost all grip on chastity, decorum, and even reality.