Having a junction table (many to many relationship) is the right way to go.
I don't know/ can't tell how the advisors are entered now or if the code is a snippet or the complete subroutine, so.........
There are a few ways to do what you want. Most require a fair amount of coding.
One way is to get the [StudentID] & [AdvisorID] values and store them in a global variable or have hidden fields on the form.
When you change an advisor, you would open a recordset with the criteria for both [StudentID] AND [AdvisorID] PKs. Then you could update the [StudentID] & [AdvisorID] values (all of this in code).
Another way is in the attached A2k mdb. It uses a subform with bound combo boxes.
If you would rather use code, it would be helpful if you would attach a copy of your mdb (in A2K format).