Nikon D3100 best setting for football game crowd?

I'm taking pictures for my school's Yearbook. I've already been taking pictures with my D3100 but when I take pictures of crowds at my school's football games the pictures turn out bad. Without flash the pictures look flat and flash makes the people look different.

What is the best lens and setting for this kind of pictures?

The "best" is a fast lens that costs way more than the D3100 body cost.

You really don't understand. Regardless of the shooting situation, which by the way is always unique,
there's no such thing as a "best" setting. It all depends on the lighting available.

You can use a fast lens, I'd suggest the Nikon 50mm f/1.8 and a tripod. If the light is less than desirable your shutter may take it's time about closing again as the sensor tries to gather enough light to make a photo. Also, I'd suggest using your camera's 10-second timer because even you pressing the shutter release and even on a tripod will cause blur… Got that?

So buy the lens but do expect some blur from the people as they move. Since the shutter may be open for a little longer than normal any audience movement will cause blur.

Also look for abnormal color which may require some editing because stadium lighting isn't the best when it comes to the audience.

Back in my day we carried 4x5 Speed or Crown Graphics and a heavy 100wt electronic flash with ISO 400 speed film, for football pictures.

So trying to equate that to today's digital cameras I would recommend the following;
1. A high ISO (which you already seem to to be using if your pictures are turning out "flat".)
2. You might see if in the menu you could use the "sport's" setting or something else that would "beef" up the contrast. This is bit of trial and error-but back in "my day" that was the best way to learn.

So that covers "settings'. Lenses you would need a fast wide angle lens Wide angle lenses have great depth of field and are used in scenic pictures primarily. You mention "crowds" so you are not trying to single out say one person, or a small group. I would stick to about a 28mm lens. Yes I know, the zoom on your camera probably has 28mm, but the speed of the lens is obviously too slow to get what you want. That's why you purchased a DSLR in the first place. To be able to photograph things that simple point & shot cameras where not capable of.