The best results I had were with a combo of stain devils (biro remover ones) mixed with many doses of the liquid vanish which I would pour on and leave to dry several times before I washed them. If the pen has only touched the shirt briefly this does tend to come out. If it's been left in contact for a few seconds then it's in the bin as even neat bleach won't touch it
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My DS wrote off 5 shirts last year and by day 2 of this year had written off yet another! Worse still our school has just decided to change our white shirts to special coloured ones which cost £5 each!! You can tell that the parent governers who haven't got a braincell between them thought that one through haven't got young kids of their own 