First of all, if you're only going to be running Windows XP, be prepared for a lot of issues. Yes, XP is still quite popular with a lot of business out there, but it is an old OS that has lost support and most all manufacturers are moving away from it or have done so years ago.
This means lack of driver support in some cases, lack of software support, and most of all a much greater vulnerability to viruses and malicious attacks. My recommendation would be plan for an upgrade to Windows 7 as soon as possible.
You are going to need a server system to manage your school network and perform several duties. If you are only going to use a single server and have it run multiple virtual machines to do what you need, you're probably looking at needing something pretty expensive. I'd recommend looking at getting at least two physical servers and configuring them as failover clustering if possible. In the even that one physical server goes down, the second server automatically takes over the roles and there is little to no down time. Otherwise, if your server goes down, everything goes down. That's not the greatest position to be in with this big of a network!
Categorize your computers as much as you can for ease of management. In other words, don't just split them into "staff" computers and "student" computers, but segment if you have computers for teachers, computers for administrators, computers for labs, computers for class rooms, etc. You can then customize your software roll out and access permission and restriction rules more fine tuned. For instance, on lab computers and student computers you may want to use a program like Deep Freeze or SmartShield to lock the system into a non-persistent environment. This means any changes made to the computer are lost once it reboots so it always goes back to how it was before.
What size of campus do you have for this school? This will play a role in what kind of networking you will need to accommodate and the hardware you will need for it. You are going to need some high-capacity gigabit switches for running all of your devices. Ideally, you would get switches capable of 10G uplinks to connect all traffic back go your backbone where your server and core internet connection will be. You will also need to figure out what you want to provide for wireless connection. It can get pretty expensive to set up a full wireless access point configuration throughout a full campus and make sure you aren't having any kind of dead spots or signal conflicts!
You are also going to have to look into additional networking hardware such as routers and firewalls. It might be best to find an all-in-one solution like a CISCO ASA or a Sonicwall NSA series firewall which can also do gateway anti-virus protection and content filtering.
What are the types of computers that you are starting with, you said that you already have 80 computers purchased and configured, correct?