'providers' => [
    'users' => [
        ...
        'model' => AppStudent::class,
    ],
    ...
 ]

...
use IlluminateFoundationAuthUser as Authenticatable; //remove this line
use IlluminateDatabaseEloquentModel; //Add this line

class User extends Model {
     ...
}

...
use IlluminateFoundationAuthUser as Authenticatable; //Add this
...

class Student extends Authenticatable {
    ...
}