Another simple Table - RoomNumber can be created containing RoomID to be used to relate two tables.
You could also use RoomNumber::TeacherName to relate to SchoolTable::TeacherName
All the Teacher's names would be entered (or imported if you have them in an Excel File, CSV (comma delimited file) or tab delimited file.
All the room numbers would be entered (or imported with teacher names if available in the import files)
Another advantage, if a Teacher's Room Number changes, the change would propagate in all Student Records.
Are you saying that a teacher always only teaches one room?
Do you have a table containing all the teachers?
Because then you just add the room field to the teacher table; Give every teacher a room number and voila.
When you add a taecher you will have added the room number. Just place the roomnumber field from the teacher table to the Student layout.