Hi W. Chung,
SG and Wayne are correct. We can't drag and swap.
Here is an idea. This example is for a small class of 9 students that may be useful for a larger class.
A class with 9 students seated in 3 tiers (A, B, C) and three ranks (1, 2, 3).

Formula in D2 (and Fill Down)
=B2&C2
Explanation of why we need Column D comes later.
Formula in E2 (and Fill Down)
=COUNTIF(D$2:D2,D)
The formula in E counts duplicates in D.
Explanation of why we need Column E comes later.
Formula in F2 (and Fill Down)
=IF(E2>1,"Boing!","")
Explanation of why we need Column F comes later.
All is good. Only one student per chair (column E). Column F (Boing!) shows no alarm.
Save your document!
Suppose that you want to swap Alan with Kevin.
Duplicate the table (Class Seating Plan-1).
Type Kevin's Tier (B) and Rank (3) into Alan's row.

Column F now shows Boing! in Kevin's row.
Boing! could be useful if you are called away and you forget what you were doing 😉.
Type Alan's old Tier (A) and Rank (1) into Kevin's row

Column E shows one student per chair and Column F shows no Boing! All is good.
Regards,
Ian.