First, take heart, it is legal and the process is less of a pain in the bum than it seems at first glance, if you want a blow by blow account of me getting cross and then calming down as I went through the process then read this first - The Legal Process in Detail.
If you have questions that aren't answered here then leave a message under the post I linked above and I'll fill in the gaps.
What you need
A cover letter explaining why you are going to home educate. Be diplomatic, in other words ... lie.
Don't tell them they are crap, say you'll be going home in a few years (plans change, nobody will hold you to it and kick you out of the country) and the kids need to be whizz bangy in the UK/USA/Oz curriculum.
This is what your cover letter should look like, copy it and put your own details in.
All’Egregio Direttore Didattico
Scuola Elementare di YOUR TOWN
I sottoscritti YOUR NAME e, OTHER PARENT'S NAME ,(SEE NOTE 1)
residenti a TOWN in STREET, CIVIC NUMBER
genitori di YOUR KID'S FULL NAME
nato a TOWN AND REGION OF KID'S BIRTH il KID'S DOB
Una situazione familiare grave ha reso molto probabile un nostro transferimento in Inghilterra. (USE MY EXCUSE, OR ONE OF YOUR OWN)
In conseguenza da START DATE OF HE abbiamo la neccesitá di educare KID'S NAME seguendo il curriculum nazionale Inglese e Italiano. (vedi allegato per dettagli) NOTE 2
Noi auto-certifichiamo di avere la capacitá economica necessaria.
Noi auto-certifichiamo di avere la capacitá tecnica necessaria..
YOUR NAME OTHER PARENT'S NAMES PLUS SIGS
YOUR NAMES AND ADDRESS
NOTE 1 – It must be the name of the kid's biological or adoted parent. A new spouse can't sign for the other legal parent. If the biological parent no longer gets to decide anything for the kids you will need to show documentation (in Italian, probably notarized) to support that fact.
NOTE 2 - Add a copy of the non Italian curriculum you intend to use, ask the school to provide you with their curriculum, unless you really fancey writing one of your own that matchs the national standards.
You also need the actual letter which you can download and fill in with your own details here.
These copies of the law to help stem the flow of objections that the whole thing is illegal one two