Allow user to write over auto-filled entry from custom calculation script

[Frank28702448dnsn]

This seems like it should be easy, but I'm just not getting it.  Simple hypothetical to capture my issue:

I have a dropdown menu where the user selects a teacher's name.

I have a field for "classroom" that autopopulates with the teacher's classroom number using: 

var selectedTeacher = this.getField("Teacher").value;

if (selectedTeacher=="Teacher1") event.value = "Room1";

else if (selectedTeacher=="Teacher2") event.value = "Room2";

. . . 

else event.value = "";

But sometimes, the teachers are assigned to different rooms.  So I want the user to be able to manually type over the room number that autopopulates (for example, to replace auto-filled "Room2" with a typed in "Room1" if Teacher2 happens to be in Room1 for the day.

Can anyone assist?

[Bernd Alheit]

Use a validation script at the dropdown.