© PAParents are also being urged to dress appropriately for school meetings and assemblies.
She said her aim was to help set a good example for pupils.
Parent Phil Naylor said wearing nightwear to school was "disgraceful".
Ms Chisholm said the final straw came when parents wore pyjamas to the Christmas show and to recent parents' evenings.
Wash and get dressed'
"It just got to the point when I thought 'enough's enough'," she said.
"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.
"I'm afraid wearing pyjamas, going to school, maybe doesn't reinforce that somehow."
In her letter, Ms 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," she said.
"If I get the parents on board then we often get the children too and in order to get the best chances for the children we have to raise the bar with the parents."
School parent Mr Naylor said Ms Chisholm had his support: "It's disgraceful, we should be guiding our children not giving them bad habits."
Whaaatt?!!
There's serious problems here that a letter home to parents just isn't going to touch.
These parents sound like they've lost the will to live. Poor sods.
Darlington's a deprived northern town, its famous engineering industry has collapsed therefore above average unemployment, whilst the population is growing faster than any other town in Yorkshire. I'm guessing there are many more 'Indian' restaurants now. And a failing infrastructure.