Sharpies WILL wash out, eventually. My kids all danced ballet for over a decade and it was the tradition to have all the cast and guest stars sign the official Tshirts/sweatshirts for each ballet... They always passed around a sharpie to do this. With each washing, those autographs faded until finally they were all gone-- washed away. Very sad!
Always use a fabric pen that says it's permanent and for fabric, and then heat set it with your iron.