In the case of a school of 2000 students our recommendation is to use the hosted version. The version installed on your server is meant for universities or large installations of Moodle (including from cost point of view).
You ask about downloading WIRIS CAS to your server and tell me that you've installed WIRIS Editor. Be aware that you have downloaded only part of the editing system of formulas. Each time you create a formula there is a call to an image web service resident in www.wiris.net
About FREE use of WIRIS tools. Our policy is not to request a license to schools making a reduced use of the service. We keep track of the use of the service since the web service is offered from our servers.