Makerbot recently promoted a document listing different grants that they claim support 3D printers in schools. You have to sign up for their mailing list, at least temporarily, but the document is several pages long and may prove of use to your school. Here’s the link to the request page.
Here‘s a grant writing guide specific to 3D printers. No sign-up required.
Here‘s a much more extensive list of grants and a link to a grant writing guide by Stratasys, the parent company of Makerbot. Also no sign-up required.