You gave yourself a real challenge shooting in that gym. There is not only the insipid yellowish cast from what it probably the old school sodium vapor lamps or similarly terrible lighting, but there is also daylight (see the bluish reflection in the bleachers) that casts blue into the image. White balance for the yellow, the blue light gets bluer, white balance for the daylight, the yellow gets yellower. Mixed white balance and skin tones can be a real pain to correct because we all have an idea of how skin tones "should" look.
In a situation like this, one needs to bring their own quality lighting and overpower the terrible lighting in the setting that the client wants in the shot. I know that may not be an option, but it is something to think about.
If you do not have the strobe power to make your own light, large group shots work well in overcast, diffuse lighting because it is soft and consistent and there are no harsh shadows and no one is squinting too terribly. Even if you are shooting relatively wide (to get the whole group in the shot) you can place them in front of a very distance scene (the trees far away from the ball field or whatever) so that there is some background blur and separation that adds some pleasing appearance to the image.
Crop for the composition, or the specific framing that is required for the display. Then you can use photoshop to enlarge the image to the correct pixel dimensions for the desired output resolution and physical size. No one is going to be looking at the fine details except you, and that might be an unrealistic gauge of the quality for the final display application. Make sure to sharpen for final output as the final step in the process - choose appropriate sharpening for the print device and the viewing distance/print size.
good luck!
Kirk
