In response to Joee's comment and question.
It is a bit of a minefield assessing what is appropriate dress for Church. Having said that, it is a very helpful exercise for any young person to consider why they dress as they do. For girls, I guess it is simpler - if you are dressing in order to arouse boys sexually then you will almost certainly succeed. Sadly, you may be "only flirting" but they will not be - they will be serious. Apart from that, on the finer question of what is appropriate and what is not, it might be helpful to talk to a sensible older women that you can trust. I thought Barbara McGuigan's comment sounded helpful - dress so that the first thing a boy looks at is your face.
For boys, I think it is a matter of self-respect and knowing what is right for a particular occasion. When Year 11s have their "Leaving Ball" the boys go in black tie (Sorry - I just can't bear the ghastly American usage "Tuxedo") and they all want to have a photo taken. They have worn something special for a special occasion and really look "the business".
Going to meet our Lord at Mass is also a special occasion. It is different because Our Blessed Lord invites all to his feast. So it is not a matter of having expensive clothes or showing off - those are also abuses of Sunday Mass. But I think it is a good thing to try to dress smartly, with self-respect, and with respect for Jesus. If a boy never wears a tie at any other time but puts one on for Mass, that is saying something and reminds him in a small way that he is going to take part in something awesome.