Pass multiple values into a t-sql 2012 stored procedure

Transact-SQL
1

In t-sql-2012, I am trying to load all the schoolID values into the variable called @CalendarID.
I then want to pass all the values into a parameter called @schoolID into a stored procedure called procCalendarS.
The dbo.fn_splitString is a string split function at my company where you can split a string of values.
The parameter going into the stored procedure called schoolid is declared as nvarchar(max).
Here is the sql.

declare @calendarID VARCHAR (8000) = (SELECT schoolID from dbo.school)
DECLARE @return_value int

EXEC @return_value = campusOPS.[dbo].[procCalendarS]
@schoolID = SELECT * FROM dbo.fn_splitString(@CalendarID),
@endYear = N'2019'

SELECT 'Return Value' = @return_value

can you show me how to pass all the values called schoolid from the school table into the @schoolID paramter?

2 
DECLARE	
	@return_value INT,
	@calendarID VARCHAR (8000);
SET @calendarID =  
	STUFF
	(
		(
			SELECT
				',' + CAST(schoolID AS VARCHAR(32)) 
			FROM 
				dbo.school
			FOR
				XML PATH(''), TYPE
		).value('.','varchar(max)')
	,1,1,'');
EXEC @return_value = campusOPS.[dbo].[procCalendarS]
	@schoolID = @calendarID,
	@endYear = N'2019';
SELECT	'Return Value' = @return_value;