Just a single license ? Otherwise, look at Microsoft Charity Open License Agreement (which generally means 3 or 5 copies minimum depending on product).
If you need more than one license, then volume licensing is worth a look because you can also license just the individual programs if need be, such as just Word.
Because they've already got a PC, you can't get an OEM licensed bundled copy.
You may potentially
be eligible for an "Education" discount if you are a public museum, organised permanently for educational or aesthetic purposes, with a professional staff and regularly exhibiting tangible objects to the public, or your charity's formally stated aims are one of the following (a) the relief of poverty, (b) the advancement of education, (c) the advancement of social and community welfare, (d) the advancement of culture, or (e) the protection of the natural environment. See here for more info.
If you want to try your luck at an Educational license, just get in touch with one of the larger Microsoft resellers (e..g. Dabs, Insight, Misco, RM Education etc. ).... unfortunately I can't name names or make any other more specific recommendations without breaching PPRuNe terms as one of my roles involves close involvement in the IT industry. I have no involvement with any of the four names mentioned above.
Otherwise I think you'll have to bite the bullet and just buy a legit boxed copy.