The Nikon 24-70mm F2.8 would be a good choice. Maybe a 50mm or 85mm prime (non-zoom).
When shooting people, especially groups (as you'll likely have people near the edges of the frame)...shooting with longer lenses tend to be a good choice. Being farther away from the group, gives a more flattening (and flattering) image. Whereas, if you get close to them, you tend to get more distortion and people on the outside will tend to look the worst. So using a longer lens (thus having to stand farther back) would be great, but you don't want to be too far away and have to shout at them etc. Of course, the closer you are, the more wide angle you'd need from your lens...so that's usually to be avoided. Although, wide angle lenses can be great for more creative group shots...but I'm guessing that you are asking about the standard class/group shots where they are all lined up in two or three rows looking right back at you.
If space is an issue, you may be forced to use a wider focal length...so that should be an option in your choice. If you do have the room to back up, shooting with a longer lens will be easier/possible.
As with anything, lighting will be more important than what lens you use.