From the Clerking Group - go to Clerks->Appointments and you'll see something like the below.
From here you can choose to either find a school and assign the clerk to it or find the clerk and assign a school to them.
To do the former, just type in the search box and find the school. Click on it and you'll see something like this:
You can now assign a clerk by clicking on the "+" button and then typing the name of the clerk (and you can remove the clerk by clicking on the "x" next to their name).
To assign a school to a clerk, click on Clerk and you'll see something like this:
Find the clerk in the list, click on the "+" next to them and type the name of the school. Click on the "x" next to the name of the school to remove it from the clerk.
All these changes apply instantly so as soon as you pick a school and link it to a clerk, that school will appear on that clerks washing line. They will then appear in the members list for that board with Admin rights and a "A member of..." explanatory note below their name as shown below: