I came up with 2 ways to handle your situation:

(Assuming entries are in Cells A1:B???)
This one allows you to enter duplicate Student IDs, but flags them with a
colored cell background:
1)Select the Input Range
2)FormatConditional Formatting
3)Formula Is: =SUMPRODUCT(--($A$1:$B1=A1)*(A1<""))1
Format: ( I set the cell background color to bright yellow)
Click [OK]

This one prevents the entry of a duplicate Student ID:
1)Select the Input Range
2)DataValidationCustom
3)Formula: =SUMPRODUCT(--($A$1:$B1=A1)*(A1<""))=1
Click OK

Does that help?
--
Regards,
Ron