White Clothes with black screen printing need to whiten them again ?

My 2 oldest girls are in Tae Kwon Do and their outfits are white of course. Well they also have black screen printing on the back of the shirts for the company. Their outfits are starting to look dingy and i want to whiten them again. I have tried bleach and oxy before the screen printing was done and it didn't work, i need something that will work but won't run the black into it or ruin the print. Any ideas?