Open manage | Value lists and click the button to create a new value list.
Enter a name for your value list and select the "Use Values from Field" option.
Select Teachers from the drop down of table Occurrences at the top, then click TeacherID to highlight this field. In the second drop down, you can select TeacherName so that the value list has two columns of values, the ID number in column 1 and the name in column 2. This value list will list all teacher names and IDs from your teachers table. If you select a value from this list, the ID number will be entered. If you enforce unique teacher names, you can even hide the first column of ID number from view.
Click OK until you've exited the Manage dialogs.
In layout mode select your TeacherID field on the students layout and use the inspector to change the field from and Edit Box to either a drop down list or pop up menu and select the name of your value list in the drop down. (You can use the pencil button here to create or change value lists as it pops open Manage | Value Lists for you.)
That worked! Thanks much!