Kate Chisholm, headteacher at Skerne Park Academy, Darlington, made the appeal after she noticed an increase in the amount of parents wearing nightwear to the school gates.
Chisholm also asked parents to stop wearing pyjamas to school assemblies and meetings.
In the letter to parents she wrote, according to PA: "I have noticed there has been an increasing tendency for parents to escort children to and from school while still wearing their pyjamas and, on occasion, even slippers.
"Could I please ask that when you are escorting your children, you take the time to dress appropriately in day wear that is suitable for the weather conditions."
She said her aim was to help set a good example for the children and said that so far she had received a positive response.
"We are trying to raise standards and get better outcomes for the children and we noticed a lot of the parents are turning up to school as well as meetings and assemblies wearing pyjamas, if we're to raise standards it's not too much to ask parents to have a wash and get dressed," she said.
"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.
"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."
Phil Naylor, a parent who has children at the school, said: "We all support the school and I hope this helps get the message across to parents.
"It's disgraceful, we should be guiding our children, not giving them bad habits."