FOR JSON PATH vs FOR JSON AUTO SQL Server
You need to use a subquery to produce a property that has a list for a value. Use an alias for the subquery that is the name of the property on the resulting JSON object.
This should do it for you:
SELECT d.SCHOOL_WEBSITE AS 'websiteURL', d.SCHOOL_EMAIL AS 'email ', d.SCHOOL_TELEPHONE AS 'phone', d.[Address 1] AS 'address.address1', d.[Address 2] AS 'address.address2', d.[Address 3] AS 'address.address3', ( SELECT [IndiUID] as 'name', Value as 'value' FROM [dbo].accreditations as ac WHERE ac.SchoolCode = d.Code FOR JSON PATH ) AS accreditationsFROM dbo.DataValues dFOR JSON PATH;(As a side note, you should discontinue use of the old implicit JOIN syntax.)