I want use Entity Method but encountered Error

Entity uses the $attributes property to store data. This storage is accessed either directly or through getters and setters.
Since you declared the properties corresponding to the fields, no getter/setter is called when accessing them. This means there is no access to the data.

Remove the properties and it will work.

PHP Code:

protected ?int $id;
    protected ?string $name;
    protected ?string $zipcode;
    protected ?string $address;
    protected ?string $tel;
    protected ?string $prefecture;
    protected ?string $school_code;
    protected ?int $school_category_id; 


You can also remove methods. They don't make sense unless you need to mutate/calculate the data.
For type casting, you can use this Property Casting