XP is dead. If you aren't stuck to a legacy system (as this guy isn't) you would be a complete fool to stick with XP. It would be a mistake you will constantly regret. Most of the things you would want to extend XP's features are built into 7/Server 2003. Remote administration, Patching, Application Control, Network Image Deployment, locking down the desktop like deep freeze does, all can be accomplished with built in (and supported) features. Security is also better (requiring drivers to be signed, built in support for full disk encryption, Memory address randomization, better default settings, better implementation of SFC, etc), and the systems are a lot more usable running as a non-admin without lots of extra scripting work. You also get better ip v6 support, and improved network performance in general. Just the fewer headaches in patching alone makes it worthwhile (even with a WSUS server, I find myself frequently manually updating XP machines, I've never once had to do it on a 7 machine).
The 7/Server 2008 networks we have deployed require substantially less maintenance then the XP networks. Support for XP is being phased out on new hardware, as it is you have to stick to certain long-term support models to get support for XP from the big OEM's (there's a difference between "heres some drivers, good luck" and officially supported). 7 is a mature OS, if it makes you feel better think of it as Vista service pack 3. Furthermore if you don't have the cash to shell out for VLC licenses, expect trouble when Microsoft drops downgrade rights on OEM licenses. Setting up a brand spanking new network with Windows XP is like making a brand new web app from scratch, and designing it in Visual Basic to only work in IE 6. You can do it, and the technology is tried and true, but you will be creating more work for an inferior result that will bite you in the ass in a short time frame. The only reason for not deploying 7 on new hardware where you are not constrained by legacy code is you want to stay in your comfort zone, and are scared to learn new things. If that's the case, you need to GTFO IT, it's the wrong field for you, and you are doing your clients/employers a disservice. Being skeptical of new technology is fine, but being irrationally afraid of it is stupid. As far as Engineering/Science goes, any commercial software package that can't run at all under 7 is probably on it's way out anyways. Whats bleeding edge today will be a generation behind by the time the students get into the real world.
All that said, I think XP/7 is the WRONG way to go. If you want a Windows environment, your best bet will be to buy some thin clients, network boot them with something like ThinStation, and have them RDP to a farm of nice beefy 2008 R2 Terminal Servers. Thin clients are the only thing I've seen hold up to a school environment. Unlike a corporate environment where you can expect the employees to only cause damage out of ignorance, high school students will be actively malicious, and will destroy/break/steal things just to do it. If you lose a thin client, the teacher can yank it out, pull a spare from the closet, and send the old one to be diagnosed/redeployed in your spare time. Because they are stateless, if one is stolen you are out a couple hundred bucks and not any information. It will be easier to setup a consistent environment, and you can shop around to different hardware vendors if needed while maintaining a consistent experience for the students. It will be easier to create flexible lesson plans, install software, and you can often really cut down on licensing costs. Thin client tech has come a long way, and if you spec your servers properly, and have a decent network, you can't tell the difference. I took a class in Solidworks (a ram hungry and CPU hungry 3d CAD program that makes your average office workstation dog slow) that was taught in a lab using thin clients and terminal servers, and it ran better on them then my personal laptop, despite having 20 other users on the same server. The instructor could easily drop files on our desktop, add or remove programs/options in advance, etc. If you ever got something really screwed up, worst case scenario was you logged out and back in. Ive since been part of several thin client deployments and I love them.
As an added bonus,it's even possible to allow students to connect to the server farm from home, so they can work from their home pc without needing a license for all the software. Another school friendly facet of the tech is you can easily swap which class is being taught in what lab, without needing to reimage the desktops to get the correct software load. Frankly for education there isn't any other way to do it that is as good, and the long term maintenance savings (1 admin managing the server farm is as good as 5 managing desktops) more then pay for the initial outlay.