Removing grass stains can be difficult. Page 175 from The Trouble with Tulip has some suggestions that really work!
1. Try dabbing the stains with rubbing alcohol.
2. If that doesn't work, try sponging on white vinegar.
3. If that doesn't work, try rubbing in toothpaste.
Launder as usual but don't dry the clothes until the grass stains have been completely removed, lest you set the stains.