I've got to say, I'm not really a fan of it. Like most of that photographer's work, it looks pretty tacky to me. I'm not terribly offended by it, though. I get why it's problematic, I get why it does offend some people, but... the only thing about it that really bothers me is that it looks cheap and trashy.
I do think that it was a bad decision to dress the women--and at 24, they are women, not girls-- in school girl outfits. They obviously don't look like they're still highschool aged, but there is the problem of sexualizing youth, and putting the image of schools as sexy out there. Yes, teens are often sexual, and the women do play sexual teens on Glee (hell, Quinn gave BIRTH. That's about as sexual as it gets!), but they are not actually their characters. The photoshoot isn't a couple of schoolgirls posing in lingerie-- it's a couple of adults posing in lingerie that is vaguely reminiscent of schoolgirl stereotypes, posed in a school setting.
Ok, so it is a little creepy. But it isn't creepy because of Dianna Agron and Lea Michele. 24 year old women posing sexually for money isn't creepy-- and it wouldn't be creepy if they were stripping or having sex for money either, although it would be a questionable career move-- but dressing adults up as kids so that you can get a sexy teen photoshoot? Well, that is creepy. But... I don't really think anyone is supposed to be fooled into thinking they are teens-- the school theme is just because of the characters they play. Still, it wouldn't have been hard-- and would have been much more interesting-- to do a photoshoot that wasn't a direct reference to the show and their characters, or that allowed them to be sexual in classier way. Afterall, the shoot was for GQ, not Maxim.