Use views to prepopulate fields I have a site that generates letters to be sent to schools. 

The visitor arrives at the front page where they are presented with a map (leaflet map) with all the local schools marked on it. They then can click on a specific school to view its page (node with school location (addressfield) info).

Here they have the option to 'generate a letter to the school' - I have the form (entityform) set up but it needs the visitor to type in the school address again. How can I dynamically fill the address-field in my form  based on the school the visitor navigated to the form from? 

map>school>school-specific-form

There are over a hundred schools, so would prefer not to have to duplicate my form...

Thanks.
If you don't want coding, this is one possibility:

If you only have one form, but hundreds of nodes, another way I would suggest to do it is to use Webform Template for the School content type. You can create one webform template and then reuse it for every school node, but the address field in the webform should have a default value that is a token of the node's address field. All you would need to do is set up your webform template's address field to have a default value of something like [node-field-school-address] and every instance of the webform will have the address of the node it belongs to. 

In order to use tokens in a webform, you will need this module: Webform Patched . This is a module that contains the Webform module itself along with token support an therefore the original Webform module should be disabled. I implemented this myself for my site and it worked without any problems. 

I never worked with addressfield, but I found this module in case you need it: Addressfield tokens