1. Will "old comps" be fast enough for XP Pro?
300mhz, 128mb ram minimum; 450mhz, 256mb ram recommended. Hdd's are recommended to be minimum of 4gb as XP pro+SP2+other stuff can take up about 3gb or more.
If all 20 are same PC, make-mfg, and add-ins, the job will be much easier. XP Pro allows many registrations if PC is almost exactly the same. Only 2 if not, unless your call to Microsoft gets cooperation.
2. I assume you will have an "institition copy" of XP Pro, if or not, you need to contact Microsoft to get advice about installing same copy of XP into 20 different machines...RE: must register online within 30 days or XP will quit working.
This may involve networking all 20 until done, with a modem in one, or installing a modem in each one, one at a time or 20 modems. Hopefully you have or can get the free MS CD with SP1-SP2 on it.
This is also about whether all 20 will need to be online-capable, and what the school uses for internet connection, Hi speed or dialup.
This is also about having an antivirus and being able to connect to keep updating weekly. If online-capable, it's also about spyware cleaners and keeping them updated.
USUALLY a school networks PC's with a main one so an administrator can do all these chores, and fixup, as students will mess them up. Student PC's are restricted about connecting with each other, and the internet, and the main server. Networking cable and NIC cards cost $$ if networking is not already on motherboard, and in building.
USUALLY, a school does not equip-allow classroom PC's to connect to the internet.
OFTEN, a school will contact a nearby college computer department for help...a college project to set you up right...free.
You'll have the same problem in 2 years to update to Longhorn. I'm assuming that the school wants to teach the current Operating Systems. If not, W98SE would be easier to deal with as the Hdd can be copied over as a slave, but lots more about that.
3. To wipe Hdd's, I use PowerMax from Maxtor, free, as it isn't proprietary, to write zero's to Hdd, or low level format. Then each needs to be repartitioned-reformatted. If all hdd's are same mfg, download the free Hdd Utility from the Mfg. Some have an option to copy a partition or whole Hdd to another.
Even with XP Pro, if you format with FAT32 beforehand and opt to keep this partioning, instead of allowing NTFS, the older W98SE programs will work. XP does allow most older programs to work anyway. What software do you want to load?
Give more info.