Looks like something has not been packed into the blend file. For Crowdrender to work properly you will need to pack all assets into the blend file. For the classroom scene this is a bit more involved than if you have just one scene with a bunch of objects, materials and so on. The classroom scene has multiple linked or appended assets from other blend files, each of which sometimes have external textures that themselves are not in the blend file.
So the way we managed to render this was to first go into each of the linked blend files, then make sure those files each have their textures packed (using pack all into blend file helps), once that is all done the last step is to pack the linked and appended blend files into the main scene. The command for this is a little bit hidden as it doesn't show up in the "external data" submenu (which is in the File menu).
To find the "pack libraries" module, you'll need to put your cursor in the 3D view and hit space bar, then you'll get the operator search tool. Type in "pack lib" which should narrow the search and show you the Pack Libraries command. When the command works you'll see a notice saying how many files have been packed. It should match the number of linked files.
If you have any further problems, please let us know :)