Please provide us with a photo or sketch of what you believe is suitable attire for a traditional Catholic girl to wear on the public volley ball court.

The issue is not ONLY "what is suitable to wear"?  But also,
1) "Whom are you playing against?  Are they dressed impurely?"  (The issue of 'bad companions' applies here).
2) Is this game private (i.e. girls only) vs public?
3) What is the context of the game?  Is this a recreational game or a school-sanctioned, diocesan-affiliated event?
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The picture posted earlier on this thread is objectively not in accord with Traditional Catholic standards.  Those who argue that "isn't it modest enough?" are asking the wrong question.  If a few inches of fabric ABOVE the knee is no big deal, then why not make the skirts a few inches BELOW the knee?  It's just a "few inches", right?  Is a long skirt really going to impede proper volleyball performance?  Longer skirts didn't impede female tennis players from being some of the best in history, even by today's standards.  So I don't see how a longer skirt could impede a volleyball player, being that tennis requires way more mobility and way more running, and for a longer duration, than does the game of volleyball.
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The picture posted is, one could argue, not gravely immoral (though it is certainly a venial sin, as it is objectively not Traditional) if viewed on its own.  However, in the context of such attire being worn by supposed-Trad girls, while in the public arena for anyone to see, and in the context of a non-emergency, recreational activity, where the opponents are ALL dressed totally and scandalously provocative, the circuмstances of this not-grave attire make the action a grave moral offense.
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Finally, even if the skirts in question were down to the ankles, the circuмstances of playing in an organized league, against competitors who are GRAVELY immorally dressed, makes the activity gravely sinful.  A man is not allowed to visit a "dance club", just to get a drink and read a book in the back.  "I'm not there for the dancers; i'm reading a book."  Such circuмstances make his action gravely scandalous and gravely sinful.  In the same way, those who participate in an activity where there is CONSITENTLY GRAVELY immoral dress, are sinning by the "9 ways" one can be an accessory to a sin, by silently condoning the dress and by not avoiding a grave occasion of sin.

Okay.. okay.

Let’s imagine we are living in the “Sixth Age of the Church”.

What’s left of the Jєωs are in hiding and everybody else is Catholic. The Restoration Pope has reiterated the rules for modest dress.

There’s a Catholic girls school volleyball league.  What might there uniforms look like?

I played football and baseball from 9 years of age till 12th grade and I was very good at it. The daily football practices and games sapped me of all of my aggression and made me a better student. Hitting people and getting hit everyday day till you can't walk will do that. Had I known the faith like my son does today (I knew absolutely NOTHING!), I would never have played, as I would not have had the aggression and the malice. It is a lot of work to play any organized competitive sport and a waste of time for 99% of the people who play it.  

Female sports today only exist for the pleasure of men. That is why girls sports are played bare butt (in pro football, track, tennis, volleyball, swimming.....). If people once again lived the faith, you would not see any girls going through all of the work to compete in sports.

If you want a hypothetical uniform for today,  I already mentioned a suggestion with a picture earlier, thinner material spandex like football pants with knee pads and hip guards so the girls don't get hip pointers when they dive for shots. Over that they can wear those skirts they are wearing now.  Hip pointers take forever to heal, because your hip keeps getting hit all season in practice. I'm surprised that those volleyball girls just wear spandex underwear on the outside. The softball girls used to wear shorts like 3 years ago, again to show butts. I thought it was insane, having played baseball myself. You slide once and your butt is scraped raw, oozing blood forever, and it will never heal while you still play. I notice now that they are wearing regular long baseball pants.

Here's what the uniform could look like, but made with lighter material with built in hip and knee padding.The skirt can be the same length as they currently use:

So the picture I see in the thread is different than what I see when trying to respond.  I think you meant the one in the response.  Not bad, but the skirt would need to be a little looser so that they can move their legs more freely, and the elbow-length sleeves just would not allow their arms to move freely enough to play well.  But this isn't too far off.  But short of having them custom designed, I'm not sure where you'd fine something like that.

Not bad, ...But short of having them custom designed, I'm not sure where you'd fine something like that.

We buy those bathing suit for our girls from Hydrochic. They last forever and can be passed on for years. The material is too thick for volleyball however,  it would be too hot. It could easily be replicated by the girls just using black leggings under their current skirts. They currently likely only have the same spandex short underwear like their competitions, and whenever they jump or bend over you can see everything under the skirt.  The leggings would fix that. A simple cure. Nevertheless, they can't play against other teams because the other girls are naked.  

No, female sports rose in popularity due to feminism ... and not the pleasure of men.  That's why even today you have on the books all these rules in school athletics about having to have the same number of sports for women/girls as for men ... with anything else being "discrimination".  Then the girls themselves are the ones who set the styles to be more provocative ... for the same reasons they do that with everyday clothing as well.

Very few men watch womens' sports, and that's why the womens' leagues usually fold and pay next to nothing.  If men are interested in ogling women, they won't be watching womens' sports, but going straight to something more provocative ... unless you count "swimsuit" as a sport (to have it appear in Sports Illustrated).

If female sports relied on women spectators, they would not last one year.  Look at the crowds at women's pro-football, which is the most ridiculous of proofs of what I say. (beware, they are really naked). 

They need men, that is why they are all naked sports. There are not enough lesbians to make a profit.

Here's what the uniform could look like, but made with lighter material with built in hip and knee padding (or they could just wear separate knee pads like they do know).The skirt can be the same length as they currently use. The sleeves above the elbow:

The misfit  could be fixed by wearing the correct size, not 1 or 2 sizes too small, as this image depicts.
Then chop off the sleeves above the elbow and hemstitch.
Insert a panel of similar fabric in matching or contrasting colour in the side seams of the skirt, and you're home and hosed.

Women have no business playing sports regardless of whether they are modestly dressed or not.

There's absolutely zero support for this emotional outburst.  You didn't even qualify this with "competitive" sports.

Thank you for your inputs.

It seems the logical conclusion to this exchange is that female organized sports are inherently evil... or a significant occasion for sin.

Therefore, in an ideal Catholic world, women would abstain from sports.