How about “It’s all right to be White and it’s great to be straight”?
I think in this case less is more.
Progress to white stickers with the simple slogan “It’s OK”. Once those have been removed and condemned, replace them with completely blank white labels.
Eventually the appearance of a blank piece of paper on a bulletin board will be branded a hate crime. Make them own their idiocy.
In fact, don’t use stickers. Just put up blank sheets of white paper with scotch tape or push pins on bulletin boards. Or just leave a white sheet of paper on a photocopier or a library table. Turn every white sheet of paper in the world into a potential Racist Object that requires the SWAT team be called in.