Cost of Windows Server Standard 2008 R2, purchased through a typical Academic Select agreement would be around £56.00
Academic CALs are also not that expensive, although it would really boil down to how the school choose to purchase - per device, or per user. Generally, for schools I would tend to recommend purchasing on a per device basis as this is something that is likely to stay relatively static, although this just a general statement provided without really looking at the school's individual circumstances, as in some cases, a per user solution can be the more cost effective option.
The important thing to get across to anybody who throws the "4GB stick only costs a few quid" argument at you is that a USB stick is very easily lost, along with the data that is stored on it. Secure USB sticks, such as the Kingston Datatraveller DTVP, Iron Key etc are significantly more expensive, but do provide an extra layer of security to your data by way of encryption. However, surely the best place to store data safely, is centrally within a dedicated SAN, where it will be secure & regularly backed up. Also remind them that if data is lost along with a USB stick, they have nobody to blame but themselves for being stupid enough to put it there in the first place.