Two questions to think off before using Static IP's or Reserverd DHCP Addresses:
How big is your school, how many clients machines do you have?
How many laptops do you have?
Where I am now, I have around 300 machines and very few laptops. This is doable. At the last school - 700ish machine, over half are now laptops. They'd find fixed IP a management nightmare.
I've never come across a scenario where I'd need static IP's on the desktop (admin staff and VNC excepted). If you use a good naming strategy then DHCP + DNS properly set up will give you all you need IMHO.