Few school pests are as resilient, as aggravating, or require such high parental involvement as head lice. Despite scientific advances, the little beasties seem invincible.
Any child, from any social background, can catch them and if the infection isn't dealt with quickly, it will spread through the school, because children will always put their heads together to gossip or plot.
Once head lice are in class, teachers also have to think twice about getting close to explain a difficult sum or comfort a small child who has just had a fall.
"I was talking quietly to this girl in my class. She is always clean, with her long hair tied back," says one P3 teacher from a medium-sized primary in the central belt. "She suddenly went rigid and was looking at something other than the book. I had to focus closely, but I realised she was looking at a small insect balanced on a single strand of hair that fell over her face. I had to fight the urge to jump back in horror. We both knew it was a louse."
Head lice are now known to build up resistance to chemical treatments, none of which kill the eggs anyway. So repeat applications are needed, but not always administered by parents. Fears that the chemicals could cause harm to children have also increased, making many parents reluctant to use them.
The result is head lice are still a persistent problem in schools.
"I probably get a note from a parent every month to six weeks, advising that their child has been infected," says Jim McColgan, headteacher at Echline Primary in South Queensferry.
"It is annoying that I can't send out alert letters any more, since the Scottish Executive changed its advice. It was always the easiest way to deal with a problem."
The Scottish Executive's line is to treat head lice like any communicable disease or infection. It argues that schools do not send out letters for other infections, such as chicken pox or impetigo.
"Alert letters also often lead parents to believe that there is an outbreak when in fact only one child in the class may be infected," says the advice.
"Those parents might then treat their own child preventatively, which is neither necessary nor advised."
This approach follows UK-wide research published by the Stafford Group in 1998. Alert letters were illogical, it said.
The report also came to the defence of schools, and the Scottish Executive agreed: regular checking of children's heads was important, but it was a parental responsibility.
"Head lice are not primarily a problem of schools, but of the community.
Stigma and tradition, however, combined with inadequate public and professional knowledge, continue to hold schools responsible," said the report.
Getting rid of the school nurse inspection, which the Stafford Group believed was "without value", has gone some way to lessening the stigma.
"I definitely feel there isn't the same stigma attached to nits the way there was in the past," says Wilma McDonald, headteacher of Dowanhill Primary in Glasgow.
"Children will now tell me quite openly in front of others if they have them and I make a point of talking about them at assembly."
Using such language as "infection" rather than "infestation" is another step, but parents still look to schools to tell them what to do.
Sending information on chemical and non-chemical treatments home by letter is the main option open to schools. Others have adopted the "bug busting weekend" approach, where letters are sent out to every parent asking them to comb their children's hair with a nit comb over one particular weekend.
It is an attempt to cull the pests in one go to lessen the chance of reinfection.
Websites such as www.nitnurse. org and www.chc.org can also improve parents' understanding.
For more chronic infections, a more proactive approach may be necessary.
"If I know a child has a persistent problem, then I approach the parents in a diplomatic way to discuss treatment," says Mrs MacDonald. "But I would never exclude the child or anything like that. It would be counterproductive."
Any action seen as punitive is to be resisted, advises the Scottish Executive. The days of sending pupils home or insisting they cut their hair or queuing for nit inspection are gone. A teacher's responsibility is limited to informing and educating. The onus for controlling this particular pest is squarely on the parent.