Where exactly is the line to be drawn? They are technically pants and they do cover the body, not exposing anything, they're just a little "Form Fitting"
Being a Catholic School, it doesn't surprise me that they'd take this approach, but I think they're going a bit far.... and the interviewed mother in the report has some good points.
It kind of reminds me of when I was in middle school / jr high, where at the time, wearing overalls with one shoulder strap unbuttoned was the "Cool" thing to wear:

Boys and Girls both wore them, yet the school started to tell people to hook on both straps or go home and change, as apparently they weren't wearing overalls in an appropriate manner and had a greater chance of falling off.
Cuz that made sense.
The school said they're just enforcing their dress code..... I'm wondering what exactly that dress code states that makes yoga pants so evil and wrong.
There are all sorts of things that could compliment or emphasize a certain area of the body, so where does one draw the line?
Maybe we should all just play it safe & maybe the Muslims have it right with the Burqa..... we should protect women from the eyes of everyone who can't seem to handle looking at something that looks anything like a human body.
At the same time, we should also protect all the males in society so that women can't see any of their body parts, especially any chance of seeing a bulge in the crotch area where their penis is.... I got a great solution for that:

I've seen girls back when I was in high school wearing jeans that gave off a huge camel toe..... so perhaps we should ban jeans as well.... or jogging pants. If someone walks, bends or does something in the wrong angle, something is going to show up for all to see.
And of course skirts & kilts.... move the wrong way or sit the wrong way and it's far more easier to see what's going on down there than what any yoga pants will show you.
Having students cover that area with a long shirt or something else to remove any sort of "Curve" to their bodies isn't any different & one step closer to personal censorship like Burqas and the full body swimsuits women had to wear at beaches back in the day.
If there were actual body parts being exposed, I could understand, but these pants are worn by women of all ages, inside and outside of yoga studios, & through every day life...... they seem quite popular around here in Melbourne. Then again, many women around here just walk around in leggings, which are even more revealing than yoga pants, lol.
It all just reminds me of the big stink mini skirts made in the 60's, yet they became common place today and nobody really cares anymore.
.... More or less.