Headteachers hold their breath awaiting the flu and chest infections which will lay staff low and bring disruption to the school and to children's learning. The stock of supply teachers will run out quickly with heads having to cover classes or take over as the only supplier of "McCrone time". It can be Easter before normal life returns with the changing clocks.
Yet, when schools are not at their best, decisions are taken which influence their further development. Strangely, it's a process of which local authorities and our other masters seem to be largely unaware and in which they play little part. Many teachers, also, are ignorant of its importance even when it happens under their noses. But during each January, headteachers and clerical staff know that they have to be on top form for the enrolment of new primary 1 pupils.
Secondary schools have a fairly accurate idea of intake numbers because children have been in the system for seven years. Village schools know likely pupils from the age of minus nine months and, since they will be the only school for miles around, parental choice hardly counts. Town primary schools are different. They do not know how many will enrol each year and parents have a real choice.
The number of children in an intake determines staffing levels for the coming year. Staff numbers directly affect school organisation - class sizes, the number of composite classes, the need for a depute head to be a class teacher and perhaps the frequency of visits from expressive arts teachers. If there are serious difficulties, staff morale can drop too.
It's still not polite to admit it but the organisational health of a town primary school, as with an independent school, can depend on a headteacher's ability to attract enough parents. And yes, that means pulling them away from other schools. If there are winners, there are losers too.
So just as schools are wilting under an attack of winter illness, they must present an attractive face to potential parents because the aim is to get them to sign on the dotted line. If you're lucky, word of mouth will have laid the ground - the best advertisement for a school is a satisfied family and it works for you long after the youngest child has left.
Next in importance not attainment results but parents' reception in school.
First contact will be clerical staff, in person or by phone, but their good work can be made or broken by the headteacher, who has to realise that first impressions count.
A mother still tells how, 20 years ago, when she arrived at the school for an appointment to discuss enrolment, I solved her frazzled state by taking over one of her twin babies for the duration of the meeting. I don't remember the event, but obviously it was more important than any of the "educational" matters we discussed. No wonder astute politicians kiss babies.
After time for questions and concerns, the best plan is to take parents on a school tour, always including primary 6 and 7 classes. Parents are impressed to find children quietly at work, looking smart and articulate if spoken to - quite the opposite from the scare stories they see in the media - and are happy to envisage their child in the same situation.
The icing on the cake is the child passing along the corridor who says "good morning, sir," in the most fawning of voices. Parents give admiring glances. So do I. Even the trickiest little toerag knows when to turn on the charm.
Teachers can help, too, by cutting a little slack to a head on flu duty.
The school's roll could be in danger. So could their jobs.
Brian Toner is headteacher of St John's Primary in Perth.