Hmm, interesting article.
Why does the school have the teachers to be the fashion police? It should fall on the parents shoulders to teach their kids how to dress properly. Of course, we know that some parents are too lenient about this dress code. So, with that being said, I can see why the school HAVE to step in to take dire measure of this situation.
Personally, I don't think it should be banned. However, it should be in moderation. There are some shirts that has it cut way deep low which seems to show the most obvious assets (that I wouldn't be ok with) but with shirts that has it slightly cut down low would be fine as long as it doesn't show most of the assets.
As for self-esteem, the girls should build confidence by being able to dress properly and yet still, look 'fabulous' without adding the 'trashy' note on it.