Any chance of a beefy server and use them as terminals?
AliExpress has some cost effective x79 / x99 Dual Xeon Mobo / CPU / RAM combos. System would be Sandy Bridge / Haswell. But 24 cores and 128gb of ram can go a long way and whole system cost would likely be under $2K USD. A few Sata SSDs and you could easily host a classroom on a single box.
Not Cheap if you don't have the money, but likely cheaper then a new laptop fleet.