Need help on MS SQL Solution

See more:

SQL

I have three tables like SCHOOL, ORIENTATION and SUBJECTS.
The tables have the following attributes

CREATE TABLE [dbo].[SCHOOL](
	[ID_chool] [int] IDENTITY(1,1) NOT NULL,
	[SchoolName] [nvarchar](50) NOT NULL,
	[AcademicYear] [nvarchar](15) NOT NULL,
	[Address] [nvarchar](50) NOT NULL,
	[ID_Person] [int] NOT NULL,
	[ID_Municipality] [int] NOT NULL,
 CONSTRAINT [PK_SCHOOL] PRIMARY KEY CLUSTERED 

CREATE TABLE [dbo].[ORIENTATION](
	[ID_Orientation] [int] IDENTITY(1,1) NOT NULL,
	[ID_School] [int] NOT NULL,
	[OrientationName] [nvarchar](50) NULL,
 CONSTRAINT [PK_ORIENTATION] PRIMARY KEY CLUSTERED

And the third table

CREATE TABLE [dbo].[SUBJECT](
	[ID_Subject] [int] IDENTITY(1,1) NOT NULL,
	[SubjectName] [nvarchar](50) NOT NULL,
	[ID_Orientation] [int] NOT NULL,
 CONSTRAINT [PK_SUBJECT] PRIMARY KEY CLUSTERED 

The problem is that the primary schools don't have ORIENTATIONS, so they don't need an OrientationName, only the secondary schools need an OrientationName.
How to solve this problem so I can register the subjects for primary schools.
P.S. My project has 15 tables, but I'm stuck in this problem and can't solve this problem by myself.
Thank you in advance for your reply.
Cheers.