I have often arrived to find no seating chart. My quickest fix is to ALWAYS have a blank seating chart among my materials (some adjusting of layout is usually required). The student roll book has the students in numbered
columns. As I call roll I simply write the number of the student's name on the correct seat. After getting them started on their first assignment I sit down and write in the names.
Also, if the teacher has assigned a class helper, they can do the seating chart for you.
For me, it is a MUST to know their names and where they are sitting ! This gives me leverage and control.
When you are able to look at a student and call them by name just moments after you have first met them surprises them !
I sub from K through 12. Most of the elementary teachers will have students names taped to the desk where they sit...if they don't, YOU DO IT !