I recognize quite a few. Alucard in the back with red eyes, under the katanas in the middle, next to the dead eyed guy* from Decoration Disorder Disconnection. Nagato to the left of that with that hair "antena" piece looking at Kaguya from Touhou doing some magic, along with one Shinonome Satsuki from Ima sugu onii-chan ni imouto da tte iitai. Shana being lifted by the necklace, with Tohno Akiha looking up at her (girl with hair band and blue eyes at the table). The two girls walking together next to the table are Semiramis from Fate/Apocrypha and Irisu from Hyouka. Going back to Isayama Yomi in the middle with the pocky, but more recognizable by the hair bangs and purple eyes. To her left, with red eyes is the girl from Amnesia talking with Yagiri Namie from Durarara, I believe. To the right of Yomi and the other purple eyed girl we see Yukino from My Teen Romantic Comedy SNAFU. Back to Alucard in the back, to the right you see Hagoromo Kitsune from Nurarihyon chatting with Hirasaka Hatsune from Atlach-Nacha (I believe). Then we see on the stairs on the far top right Yuuko from xxxHolic admiring the paintings and that girl from Accel World showing off some butterfly magic to somebody. Next to them is Tomie from Junji Ito's manga, Tomie (yes, she's creepy, it's Junji Ito after all). Up on the balcony, top left of in the image, we see Yumeko from Kakegurui wearing the standard red school uniform. Back to the general image, bellow Hatsune and Hagoromo Kitsune talking we see Enma Ai from Hell Girl showing one of those voodoo dolls to, I believe, Shionji Yuuko from Kamisama no Memo Chou. Even though she looks familiar, I can't put a name to the girl raising her arm. To her right though, we find a girl smiling, she's from Amagami.

I'm not sure where to put Lucifer from Umineko. She might be the one up front in the image (with red eyes). Or if you look closely, one is not like the other. One is wearing a red dress, and Lucifer is known to wear a red uniform. She might be talking with Yomi. You see the dress colour if you look between the two upfront/foreground girls.

I'm still trying to find Sakurako (the girl obsessed with bones), but I can't be sure between two people. First, she has a mole. That mole is important. And I can't see it on either of the two candidates. One, she uses gloves so I first assumed the one next to Akiha at the table. But then there's no pattern on her gloves. So I went bottom right and found somebody very similar, but lacking or with a hidden mole. The only reason I'm to believe it's her, is because her bangs are suddenly cut straight in the middle.

Oh also, the other purple eyed girl, next to Isayama Yomi in the middle with that pocky, is Hazuki Mina from Darker than Black.

Usami Haru from G Senjou no Maou might be the one on the far right, in the foreground. Yes, the one you can't see the face of. What gives her away is that I can't put a match with the others. The fact that there's a piece of hair sticking way away from the head, to her left, is a little giveaway.

I believe that Kamura Reiri from Kaibutsu Oujo is talking with Alucard in the back, middle. But I'm not sure....

You can also see Amamiya Yuuko outside the window, wearing her usual clothing (black winter clothes with white puffs).

And holy crap did it take me forever to identify Yumiko from Grisaia. She's sitting on the pilot seat of the helicopter.

I'm unsure where Tantei no Tantei fits in, because the woman doesn't have a hime cut. At least, in the first chapters of the magna? I don't know.

Lastly, we see the beautiful Touma Kazusa from White Album 2 at the piano.

I might have tagged Shibuya Rin wrong. I've only added her there because she has a connection with the Dia girl from Love Live. It's obvious what the connection is. The girl was previously tagged with another because the awful resemblance with said character, but then I remembered that Rin also wore earrings. So there you go.
But then the girl next to Mio always catches my eye. To the left, right before the image ends. I can't tell...

But then, it might be right. The girl next to Kuroyukihime marks the same resemblance. Even more so with the hair fringe on the left being slightly parted. So I guess I corrected that.

Marked the girl in the back of the helli as the woman from Tantei no Tantei. She's the only one that matches the hair style of Sasaki Rena.

Marked the girl that is close to Utaha and next to the piano as Shirahane Suou. She is known in the game to play the piano and the slight hairstyle on the sides matches a few game CGs.