“The problem of fashion consists in the harmonious reconciliation of a person’s exterior ornamentation with the interior of a quiet and modest spirit.” Pope Pius XIIThe Forgotten Virtue: Modesty in Dress
This is a very good article on modesty and I hope you take the time to read it and pass it along to others.
I have posted on several occasions on the need to dress modestly. This applies to both men and women; boys and girls.
I would also add that we should always dress properly for Mass.
Even though a loose T-shirt is considered "modest", it can be a cause for distraction to others. For example, last Saturday evening we sat behind a very nice Catholic family. The teenage boy was respectful and quiet. However, his T-shirt was very distracting. It was a humorous T-shirt which would have gotten a big laugh out of my family and me in any other setting.
When our son was younger he did not like the fact that we made him dress up for Mass. Here in Hawaii that means an Aloha shirt, dress pants and dress shoes. He thought everyone (translation: his peers) would stare at him. Now at 18 years of age, he dresses properly for Mass on his own without his parents having to remind him of what is and what is not appropriate for Mass.
H/T to Sue of Half the Kingdom