In CakePHP2
class Teacher extends AppModel {
public $useTable = 'users';
}
But you should be using relationships to distinguish your different user types, you can do this by simply having the Users table with the core information, then create a UserType table linked to specify the type, i.e.
Users Table
id, user_type_id, username, password, created, modified
1, 1, Stephen, somehash, 2015-04-15 00:00:00, 2015-04-15
00:00:00
2, 2, Alex, somehash, 2015-04-15 00:00:00, 2015-04-15 00:00:00
3, 2, Jane, somehash, 2015-04-15 00:00:00, 2015-04-15 00:00:00
UserType Table
id, name, created, modified
1, Teacher, 2015-04-15 00:00:00, 2015-04-15 00:00:00
2, Student, 2015-04-15 00:00:00, 2015-04-15 00:00:00
Looking at the above data example, you will be able to see that Stephen is a Teacher, Alex is a Student and Jane is a Student.
This keeps it central, you won't need to write model methods in more than one place, these links have more info on relationships.
CakePHP2: http://book.cakephp.org/2.0/en/models/associations-linking-models-together.html
CakePHP3: http://book.cakephp.org/3.0/en/orm/associations.html