Figure out how much RAM is needed. Then hit up eBay.
For workstations, search for Optiplex MTs with XGB (RAM), use the advanced search to limit results to listing which have qty: 20+ available, and after trying a few different model numbers you’ll find which generation/model is the sweet-spot. About 4 years ago, I did this and found Optiplex 8GB 3020s were about $80 each and available in huge quantities. Ended up ordering HUNDREDS of those. They are probably not the cheapest and most easily available (in large quantities) model today. I’m not motivated to spend a couple hours researching that right now.
Throw away the slow old hard drives they (may) come with. Figure out how much disk is needed and buy some new SSDs; dramatically improves performance. Be sure you’ve checked UserBenchmarks to ensure you aren’t saving $1 or 2 only to get something that has terrible sustained write performance. These $14 120GB drives will probably do.
Those systems will all be licensed for Windows, so grab the Win10 ISO from Microsoft and start installing. You won’t even be prompted for a license key.
For servers, just stick with the most common R7x0 family. R730 servers with 128GB of RAM may be under $500/ea. They might come licensed for an old version of Windows, but perhaps not, so you should plan on a new server license as well.
Then decide how much storage your servers need and look-up the specs on old SAS SSDs, planning for a RAID10 and some spares. I can vouch for these $120 Sandisk 1.6TB SSDs being great as long as you first firmware update them. HGSTs work great, too, except beware the IBM branded ones which need to be re-provisioned with WD Hugo to 95% of their listed size. In either case, you may find you get SAN (usually EMC) model drives with 520b sectors instead of the usual 512b. It’s easy to change this with sg_format on a bootable Linux, just search the forums for the details.
The benefit to this nice cheap used equipment is that you can afford to get some spares. With identical hardware, you can swap the hard drives from a failed system into a spare with no trouble (ESXi just needs one command run on the new system before it’ll see the data store on the hard drives, Windows / Linux will just boot right up). Similarly, you’ll have lots of spare parts… PSUs, fans, etc.
This assumes the school has at least a couple technical staff members who can run burn-in testing on the used equipment when it arrives, and swap out failed systems. If you really have no technical hands available, contracting out the job of testing and maintaining the equipment can be prohibitive, and buying vendor-supported (new or used) equipment with a multi-year maintenance contract can be the better way to go.