There are a few ways to do this.
The easiest way would be to create a 3rd table to setup a many to many relationship between Clients and Schools. This table would have one column for ClientID and one for SchoolID, and one row for each Client/School combination. When a client selects a School from your choice component, you would need a Submit button with an action that add a row to that table with the SchoolID and ClientID. You can then create a multi-relation between your Clients and Clients-Schools table joining the ClientID in each.
The downside with this method is that it can use a lot of rows if you have many clients and/or schools.
Another option is to use a technique referred to as the trebuchet method to create a column containing a joined list of ClientID’s in your Schools table. This option requires much less rows, but is a lot more difficult to implement.