plsql sum another table values to joined tables

You can use analytic functions rather than GROUP BY and aggregating:

select s.studentname,
       s.tax,
       invoiceamount,
       SUM(d.amount) OVER (PARTITION BY s.invoiceno) AS inv_amt_calc,
       d.product,
       d.amount,
       SUM(CASE WHEN c.code IS NOT NULL THEN d.amount END)
         OVER (PARTITION BY s.invoiceno) AS taxexclude
from   student s
       INNER JOIN studentdetail d
       ON s.invoiceno = d.invoiceno 
       LEFT OUTER JOIN specialcodes c
       ON (c.code = d.code)

Note: You can (and should) calculate the invoice amount from the studentdetails table rather than duplicating the data in the student table and violating Third-Normal Form.

Which, for your sample data, outputs:

STUDENTNAME	TAX	INVOICEAMOUNT	INV_AMT_CALC	PRODUCT	AMOUNT	TAXEXCLUDE
Paul	500	1950	1950	rubber	350	450
Paul	500	1950	1950	pencil	100	450
Paul	500	1950	1950	bag	1500	450
Georghe	1000	6850	6850	tv	1500	350
Georghe	1000	6850	6850	wheel	5000	350
Georghe	1000	6850	6850	ps	300	350
Georghe	1000	6850	6850	mouse	50	350
Mary	1500	1900	1900	bag	1500	400
Mary	1500	1900	1900	keyboard	400	400
Messy	2000	7050	7050	mouse	50	50
Messy	2000	7050	7050	car	7000	50

If you really want a version using GROUP BY then:

SELECT s.studentname,
       s.tax,
       s.invoiceamount,
       SUM(d.amount) OVER (PARTITION BY s.invoiceno) AS inv_amt_calc,
       d.product,
       d.amount,
       t.taxexclude
FROM   student s
       INNER JOIN studentdetail d
       ON s.invoiceno = d.invoiceno
       LEFT OUTER JOIN (
         SELECT invoiceno,
                SUM(amount) AS taxexclude
         FROM   studentdetail
         WHERE  code IN (SELECT code FROM specialcodes)
         GROUP BY
                invoiceno
       ) t
       ON s.invoiceno = t.invoiceno;

db<>fiddle here