My DD6 is now wearing the same white polos that her elder brother started school with in 2002. They still look almost new.
White is actually a good choice, because you can BLEACH it. However, that isn't the real secret: the real secret is ScotchGard spray. Not only does it repel stains, but it also helps to preserve the color of the fabric, helping to prevent yellowing of whites. Put it on when the shirt is new, and it will last for years with proper laundering, unless it gets torn.
Renew the Scotchgard coating every six months, and the shirts will stay very nice. Even ketchup, mustard and chocolate will come out with no problems. Do the pants, too; for grass stains, food and paint spills.
If you do get a stubborn stain, use Zout stain spray; great stuff.