There are those bleach pens in grocery stores etc. as well as for colors. They aren't expensive and they do work. You could also make a paste of baking soda and warm water. Make sure it is pasty and not watery. Apply it to the stain, let it sit for about a half hour and put it in the washer on warm water, not cold. Once washed do not put it in the dryer, let it hang dry or lie it on a towel. Toothpaste works as well, apply it to the stain, spread it around, let it sit and again repeat the washing instructions. You may find the toothpaste works quicker. I actually used it on a pen mark on a white shirt and scrubbed it a bit, even use an old toothbrush, let it stand and wash.
Good luck!! You won't have to throw your shirt out!!