Other than everything that has already been mentioned I would make sure you have the passwords and usernames for everything you need access to, and that they work. Better to discover sooner rather than later that your missing an important password to a website filtering system or wireless controller etc.
As above really, make sure you've got the passwords you need, and then for the 1st couple of months I went and got to know the system, gradually adapting and changing things and then waiting to see if anyone complains.
I probably took to much of a softly softly aproach, if I was doing it again I would be slightly more forceful, however having moved to Windows 7 last summer it gave me the opportunity to change a hell of a lot. (3 Years and when I changed some things people complained to much so changed them back. Changed them again in the Windows 7 upgrade and nobody has said a thing other then 1 member of staff who I placated)
Check what documentation is there, try and get an understanding of the rest of it and make friends with the important people who can make your life easier. (business manager/bursar, few heads of department and the reception/office staff) ;)
Congratulations and thanks for the shout-out.
the most important thing I can say to you is - if this is a new school that has never had an IT tech - they will not know what you can and can't do. Which is very reassuring. :)
My advice is to be in the staff room at break times. Listen and chat. Prove that you are a human being and not a cave dweller. It makes all the difference :)