I think that their coach is the bigger issue.  She is not playing and need not be dressed that way.

As for the girls, I think that the clothes might be as modest as you can get while playing the sport.  As far as girls' sports, if they are in a closed venue with a known girls sport taking place, then if some guy shows up there and finds himself "tempted," then he should have known better than to go there.  But it looks like they are outside of the sports venue, and the picture should not have been taken that way.  They could have had them stand up and capture only the view above the bottoms of their uniforms.  So, for instance, if there's a private property, a camp, for instance, where it's just girls in attendance, they could go swimming.  So what might be OK in that context, may not be outside of it.  Same thing applies to some girls' sports.  In a Catholic society, girls' sporting events should likely be offlimits to men and boys ... except as necessary.  But, as it is, if they restrict their immodesty to within the venue, nobody is forcing any man to attend.  If I walk into a womens' locker room and see nude women, is that their fault and are they being "immodest"?  I should not have gone in there ... obviously.  Likewise, if there's a girls' swim meet, I should not go in there, nor a girls' volleyball game.

So, the rules, IMO, should be:
1) dress as modestly as possible given the sport if someone is playing (coach and other staff do not count)
2) keep the objectively-immodest dress within the venue (don't venture outside of the venue or take pictures that aren't properly cropped)

As far as girls' sports, if they are in a closed venue with a known girls sport taking place, then if some guy shows up there and finds himself "tempted," then he should have known better than to go there. ….In a Catholic society, girls' sporting events should likely be offlimits to men and boys ... except as necessary.  But, as it is, if they restrict their immodesty to within the venue, nobody is forcing any man to attend.

Essentially what you are saying is that a group of girls could sit around in thong underwear and practice knitting and embroidery, so long as other “men” are not around, and any pics of them in the inappropriate attire are carefully cropped.

Most sports leagues require that children have practice and/or games on Sunday morning, ...

I appreciate your honest rejection of both the norms for Catholic modesty, and Catholic femininity.

Yes, I can well imagine the BVM wearing these outfits and playing volleyball.

NOT!!

Yes, I can well imagine the BVM wearing these outfits and playing volleyball.

This distinction between the precepts of the Gospel, which are binding on all, and the counsels, which are the subject of the vocation of the comparatively few, has ever been maintained by the Catholic Church. It has been denied by heretics in all ages, and especially by many Protestants in the sixteenth and following centuries, on the ground that, inasmuch as all Christians are at all times bound, if they would keep God's Commandments, to do their utmost, and even so will fall short of perfect obedience, no distinction between precepts and counsels can rightly be made.
...
The difference between a precept and a counsel lies in this, that the precept is a matter of necessity while the counsel is left to the free choice of the person to whom it is proposed.

There are many things that Our Lady would not do that are, nevertheless, licit and not sinful ... e.g., getting married and having marital relations.  Moral theologians always distinguish between the evangelical counsels, which are things that are not strictly required for all, but nevertheless represent the striving for a higher state of perfection.

http://www.newadvent.org/cathen/04435a.htm

Our Lady, of course, freely chose to adopt and to follow, EVERY such counsel for perfection, but that does not mean all are bound, nor are all capable of keeping them, so that, for instance, as St. Paul states, some need to marry rather than to "burn".