Check if a column contains text using SQL

Leaving database modeling issues aside.
I think you can try

SELECT * FROM STUDENTS WHERE ISNUMERIC(STUDENTID) = 0

But ISNUMERIC returns 1 for any value that seems numeric including things like -1.0e5

If you want to exclude digit-only studentids, try something like

SELECT * FROM STUDENTS WHERE STUDENTID LIKE '%[^0-9]%'