Though this is not a good method like D Stanley said, I would say you could write a scalar-valued function to that takes the top values from these tables and combines to form your desired value
CREATE FUNCTION GetNewID
(
)
RETURNS NVarChar(15)
AS
BEGIN
-- Declare the return variable here
DECLARE @result NVarChar(15)
DECLARE @SchoolId int
,@ClassId int
,@StudentId int
SELECT TOP 1 @SchoolId=ID FROM SchoolTable
SELECT TOP 1 @StudentId=ID FROM StudentTable
SELECT TOP 1 @ClassId=ID FROM ClassTable
SET @RESULT = CONVERT(NVARCHAR,(@SchoolId+1)) + CONVERT(NVARCHAR,(@ClassId+1)) + CONVERT(NVARCHAR,(@StudentId+1))
-- Return the result of the function
RETURN @result
Then use this function dbo.GetNewID()
to get the latest ID