Kate Chisholm, the Head Teacher of Skerne Park Academy, Darlington, Durham, made the appeal after she noticed more and more adults wearing pyjamas at the school gates as well as at meetings and assemblies.
She said her aim was to help set a good example for pupils. Chisholm said the final straw came when parents wore pyjamas to the Christmas show and to recent parents' evenings.
"It just got to the point when I thought 'enough's enough'," she was quoted as saying by a media.
"I'm not trying to tell people what to do with their lives, but I just think having a really good role model first thing in the morning, getting yourself up, getting yourself dressed, ready for business, out to school is a really good example to set," she said.
"I'm afraid wearing pyjamas, going to school, maybe doesn't reinforce that somehow," she added.In her letter, Chisholm wrote: "If we're to raise standards it's not too much to ask parents to have a wash and get dressed.
I have had loads of support from the community and people saying it's about time something was done. I have had far more positive responses than negative.
" Parent Phil Naylor said wearing nightwear to school was "disgraceful".Naylor said Chisholm had his support, "It's disgraceful, we should be guiding our children not giving them bad habits."
Skerne Park Academy is not the first school to have problems with dishevelled parents. In 2011, 11 schools across Middlesbrough wrote to parents, asking them to dress appropriately for the school run.