SQL Server – High Performance Count with Where Clause and Dynamic Data

Take your most complex (most GROUP BY columns) then use the OVER clause

SELECT
   school, board, col3, col4, ...,
   cnt_whoPassedA, ...,
   SUM(cnt_whoPassedA) OVER (PARTITION BY board) AS cnt_whoPassedA_ByBoard,
   SUM(cnt_whoPassedA) OVER (PARTITION BY school) AS cnt_whoPassedA_BySchool,
   SUM(cnt_whoPassedA) OVER (PARTITION BY school, board) AS cnt_whoPassedA_BySchoolBoard,
   ...
FROM
(
select 
 school, 
 board, 
 col3, col4, ...
 cnt_whoPassedA = SUM (CASE WHEN A.exam_PartA_score > 50 THEN 1 ELSE 0 END)
 cnt_whoPassedB = SUM (CASE WHEN A.exam_PartA_score > 50 THEN 1 ELSE 0 END)
from MyBigTable as A
group by A.school, A.board, A.col3, A.col4...
) T

However, I would consider doing this in my report layer (say in SSRS): do you need all levels of grouping at the same time?

If you don't need the other data from the NETWORK_STATUS table, how about:

select *
from dbo.NETWORK AS n
inner join dbo.vwNETWORK_KEYMSTN AS km 
  on n.Network_ID = km.Network_ID
inner join dbo.vwAPPROVAL_LATEST AS a 
  on n.Network_ID = a.Network_ID
inner join dbo.APPROVAL_VINTAGE  AS av 
  on a.Approval_ID = av.Approval_ID 
  and km.Milestone_Type_ID = av.Milestone_Type_ID
inner join dbo.NETWORK_MILESTONE AS m 
  on A.Approval_ID = m.Approval_ID 
  and km.Milestone_Type_ID = m.Milestone_Type_ID
inner join dbo.REF_MILESTONE AS rm 
  on km.Milestone_Type_ID = rm.Milestone_Type_ID
WHERE EXISTS
(
  SELECT 1 FROM dbo.NETWORK_STATUS 
    WHERE Network_ID = n.Network_ID
    and Status_Type_ID = 2
    --and Status_Type_ID = rm.Status_Type_ID
);