atcheck/database/seeds/ClassesTableSeeder.php

<?php

use Illuminate\Database\Seeder;

class ClassesTableSeeder extends Seeder
{
    /**
     * Run the database seeds.
     *
     * @return void
     */
    public function run()
    {
        DB::table('classes')->insert([
            'subject_id' => 1,
            'date' => '2019-12-02',
            'mode' => null
        ]);

        DB::table('classes')->insert([
            'subject_id' => 2,
            'date' => '2019-12-02',
            'mode' => null
        ]);
    }
}