Here's a Fashionista interview with the costume designer. tl;dr: They ARE prom dresses.
From here forward I won't be snarking the female leads' clothes because there is absolutely no point. Modern, meant to be modern, nothing to see here, moving along.
edit: From p. 3 of the interview.
Which current labels are your go-tos for working into the costuming?
I knew from the beginning that I’d have an easy time weaving in contemporary accessories. It’s funny how tiaras and hair pieces are everywhere right now, and it’s incredibly helpful. We’ve also used quite a bit of Free People for the girls’ everyday looks. They have such a strong and cohesive story with their bohemian romantic look, it’s really worked in our favor. On the pilot we used an incredible Basil Soda gown and we’ve continued using a couple gowns of theirs on Mary. I’ve rented a couple McQueen gowns as well. We shop quite a bit of vintage here in Toronto, but I’m also constantly scouring the web. Net-a-Porter, The Outnet, and BHLDN are my go-tos.
... how did you update looks from the Elizabethan era for a teen drama?
From the beginning the creators, the director, and the studio said they wanted to incorporate a contemporary feel in the costumes. The vision was there even before I signed on; I just helped execute it. But knowing the network and the show’s demographic I felt it made complete sense. I also wanted each look to have a nod to the proper period costume, whether it was achieved through a similar shape or detailed embroidery. The girls almost always wear a corset unless there’s just no need. They love how it helps with their posture. [it. mine]
Wait, corsets—did you use authentic ones?...
...We usually decide for every new look they wear if we need the corset or not. We’ve also been making embroidered and jeweled corsets to be worn on camera as day looks and formal looks for the girls.
I knew from the beginning that I’d have an easy time weaving in contemporary accessories. It’s funny how tiaras and hair pieces are everywhere right now, and it’s incredibly helpful. We’ve also used quite a bit of Free People for the girls’ everyday looks. They have such a strong and cohesive story with their bohemian romantic look, it’s really worked in our favor. On the pilot we used an incredible Basil Soda gown and we’ve continued using a couple gowns of theirs on Mary. I’ve rented a couple McQueen gowns as well. We shop quite a bit of vintage here in Toronto, but I’m also constantly scouring the web. Net-a-Porter, The Outnet, and BHLDN are my go-tos.
... how did you update looks from the Elizabethan era for a teen drama?
From the beginning the creators, the director, and the studio said they wanted to incorporate a contemporary feel in the costumes. The vision was there even before I signed on; I just helped execute it. But knowing the network and the show’s demographic I felt it made complete sense. I also wanted each look to have a nod to the proper period costume, whether it was achieved through a similar shape or detailed embroidery. The girls almost always wear a corset unless there’s just no need. They love how it helps with their posture. [it. mine]
Wait, corsets—did you use authentic ones?...
...We usually decide for every new look they wear if we need the corset or not. We’ve also been making embroidered and jeweled corsets to be worn on camera as day looks and formal looks for the girls.
From here forward I won't be snarking the female leads' clothes because there is absolutely no point. Modern, meant to be modern, nothing to see here, moving along.
edit: From p. 3 of the interview.
I think it will be quite obvious to our viewers that we’re not out to replicate historical costumes. We’ve created our own distinct look and I think viewers will respond to it positively. I’m hoping it’s an inspiration for our female viewers to creatively add to their wardrobe. It’s such a do-it-yourself kind of look. We’re constantly taking vintage pieces and dying them, altering them, beading them—all to make them our own. That’s what it’s about. It’s not for everyone though. If you’re hoping for hip rolls and men in tights it’s not your show.
Fair enough.