I think there are 3 ways to achieve this in current scenario

1. Increase the size os column [StudentName] to varhcar(400), in [dbo].[StudentDetails] table same as in extended table, trigger should work fine

2. Rather then writing insert into studentdetail table, write in extended table and write instead of trigger on this table. and in the trigger insert in studentdetail table. at this point you'll get full string that is passed.

3. Instead of using trigger use stored procedure and pass values as parameter and insert in both tables separately.

hope these options will work. according to me option 3 is best and extendable

Avinash