Do you have a 'protection filter' on your lens?
All the posted photos are under exposed (at least a full stop), and the light quality is poor.
Ideally, facial highlights should be at about 235 in the Red channel. In the middle photo the brightest highlight, a spot on her nose, is at 184 in the Red channel.
The brightest highlight on her chin is at 162 in the Red channel.
I'm looking at the photo using the cursor and zooming into the photo in ACR (Adobe Camera Raw) which allows us to see the Red, Green, and Blue (RGB) values at any point in a digital image.
What metering mode did you use?
The position of the catch lights in her eyes in the middle photo is not flattering.
Direction & Quality of Light: Your Key to Better Portrait Photography Anywhere
In the shot with the 4 of them the girl in the back is underexposed the most, likely because she is furthest from the window.
Light intensity changes as a square function. If the distance is doubled to what you want to light, only 1/4 as intense light gets to the further position of the subject.
If the distance is 4x the intensity of the light source is 1/16.
Or if you have 2 subjects at different distances from the light source, the one further from the light source gets less light.
At the least a reflector could be used to supply light from a 2nd direction and to provide some facial mask modeling shadows to promote a stronger sense of depth.
Lighting from a strobe (flash) or even constant 5500° (sun light equivalent color temperature) lighting (both natural lighting) give the photographer the most control of the light direction and quality.
On-Camera Flash: Techniques for Digital Wedding and Portrait Photography
Off-Camera Flash: Techniques for Digital Photographers