This is something that you will need to answer. Your version, settings, data, schema, hardware, etc… will affect results. Also, as the left 2 columns look like schema joined data, I am unsure of your intent.

Disclaimer: This answer assumes you are trying to count student rows while returning data. The information is still useable for general tuning.

Useful Tools: Statistics (time, io, profile), Client statistics, Execution plan

Results -- on my machine with my fake data Order of performance: 2, 3, 1

Generally, I found 2 and 3 very close with 2 offering less CPU time (statistics time -- on the messages tab). Interestingly, 2 and 3 had the same query plan for 2,000,000, but the Gather Streams (visible with Execution Plan) portion was much cheaper on query 2.

The results did bounce from time to time and the order was not always the same. Query 3 at 100,000 was using a poor query plan.

When only asking for thousands or less rows, there was no significant difference for me to choose one over the other.

Time/IO: Here you can see why query 1 is generally slower. Many more scans and reads. The query plans were also more complex.
| 2,000,000 Rows | Time | IO |
| ------------ | ------------ | ------------ |
| Query 1 | CPU time = 2906 ms, elapsed time = 7612 ms. | Scan count 26, logical reads 71430 |
| Query 2 | CPU time = 265 ms, elapsed time = 71 ms. | Scan count 9, logical reads 7143 |
| Query 2 | CPU time = 31 ms, elapsed time = 6515 ms. | Scan count 1, logical reads 7143 |
| Query 3 | CPU time = 627 ms, elapsed time = 6795 ms. | Scan count 9, logical reads 7143 |

SQL Queries: (using statistics time, io)

DECLARE @totalActive int;
DECLARE @totalstue int;

SET STATISTICS TIME, IO ON
-- Query 1
;WITH StudentCountCalculation AS 
(
SELECT ColID, ColName, Active, StudCount.TotalStudCount
FROM #tmp_dataTable
    OUTER APPLY(
        SELECT COUNT(ColID) AS TotalStudCount
        FROM #tmp_dataTable
    ) StudCount
)

SELECT ColID, ColName, Active, TotalStudCount, (SELECT COUNT(ColID) FROM StudentCountCalculation WHERE Active = 1) totalActive
FROM StudentCountCalculation
    
    
-- Query 2
SELECT 
    @totalstue = COUNT(ColID),
    @totalActive = SUM(CASE WHEN Active = 1 THEN 1 ELSE 0 END)
FROM #tmp_dataTable

SELECT
    ColID, 
    ColName,
    Active, 
    @totalstue AS [TotalStudents], 
    @totalActive AS [TotalActive]
FROM #tmp_dataTable

-- Query 3
SELECT 
     ColID
    ,ColName
    ,Active
    ,COUNT(ColID) OVER() AS [TotalStudents]
    ,SUM(CASE WHEN Active = 1 THEN 1 ELSE 0 END) OVER() AS [TotalActive]
FROM #tmp_dataTable

SET STATISTICS TIME, IO OFF

Fake Data: (Based on student rows; the active is random)

create table #tmp_dataTable(ColID INT, ColName VARCHAR(200), Active INT)

DECLARE @num int = 1
WHILE(@num <= 100000)
BEGIN
    INSERT INTO #tmp_dataTable(ColID, ColName, Active)
    VALUES
        (@num,   'Student', CAST(rand()*10 as int) %2),
        (@num+1, 'Student', CAST(rand()*10 as int) %2),
        (@num+2, 'Student', CAST(rand()*10 as int) %2),
        (@num+3, 'Student', CAST(rand()*10 as int) %2),
        (@num+4, 'Student', CAST(rand()*10 as int) %2),
        (@num+5, 'Student', CAST(rand()*10 as int) %2),
        (@num+6, 'Student', CAST(rand()*10 as int) %2),
        (@num+7, 'Student', CAST(rand()*10 as int) %2),
        (@num+8, 'Student', CAST(rand()*10 as int) %2),
        (@num+9, 'Student', CAST(rand()*10 as int) %2)
    SET @num = @num+10;
END

Closing Thoughts: Test your queries and try different combinations. If you are educated on indexes -- adjust current ones or add new ones. If you are only trying to get a small number of data/count results, pick a query for its clarity and ease of maintenance.

Lastly, if you have a totally different intent, please edit your question. Regardless, the above -- along with the comments that you have already received -- should offer insight.