There has been a practice run already, with teachers entitled to one and a half hours since August 2004. There has been positive feedback from teacher unions - and even a feature in The TES Scotland. So why nightmares? It all seems simple. Employ extra teachers to cover the classes and everyone's happy.
Well, perhaps. Teachers are happy. They have their preparation time and the support of heads who consider the time allocation to be long overdue. Some heads are happy they have acquired an additional and reliable teacher. But in many primary schools, the McCrone requirements have been a burden, with the problems hidden under an official blanket of self-congratulation.
"Robbing Peter to pay Paul" describes the plight of many heads and deputes who struggle to provide teachers with cover and achieve it only through the sacrifice of their own time. Extra teachers are not easily available, especially if it's quality you want - someone who will cope with frequent changes of class and the maintenance of good learning and good behaviour.
If you don't find this elusive person, help is at hand in the do-it-yourself advice issued by some of our experts to heads left in the lurch.
Worst "helpful" suggestion is the assembly. The head or depute takes a 45-minute assembly to allow teachers part of their preparation time. The more children can be crowded into the hall, the more teachers are freed.
"Use the assembly for 'citizenship' activities," runs the advice, and for awarding stickers and certificates too. Sing a song, play a game.
You could give them a row also, but that's just me - no one in an official capacity would suggest anything so politically incorrect. At least the row would allow me to vent my frustration.
It's a lonely and frightening experience standing in a school hall, like Gulliver, the sole giant among 400 Lilliputians. The solidarity and boost to one's authority normally provided by other teachers is absent so the possibility of being overpowered by the Lilliputians and pinned to the ground is real in my imagination.
"Get their interest and you're winning," I remind myself, but my prepared episode of the adventures of St Paul seems inadequate in the face of 400 stares demanding: "Go on - entertain me."
My spirits sink further when I notice the disruption in the middle of the massed midgets. The perpetrators ("perps", if you've been reading too many crime novels) are clever so you can't spot them, and drawing everyone's attention to the unscheduled entertainment will just hasten the downward trajectory of the assembly. Underneath it all is resentment.
The age range is too wide, the numbers too large. I know that any worthwhile assembly centres on the school community and must include teachers as important members of that community. I know I'm babysitting, wasting my time and the children's time. But I am achieving the object of the exercise, the teachers have had their 45 minutes.
Usually I enjoyed assemblies and believe that they bring many benefits to a school's life. We all lose when they are a source of frustration and resentment and exclude teachers.
McCrone's arrangements are meant to enhance children's learning. With their class teachers absent for 10 per cent of the pupil week, it falls to headteachers to ensure that this does not affect children negatively. Roll on the day when heads can choose extra staff from a pool of readily available teachers of quality. Until then, nightmares will be common.
Brian Toner is a former primary headteacher.