administraCursos/database/seeds/ProfessorSemesterSeeder.php

<?php

use Illuminate\Database\Seeder;
use App\Professor;
use App\Semester;

class ProfessorSemesterSeeder extends Seeder
{
    /**
     * Run the database seeds.
     *
     * @return void
     */
    public function run()
    {
        Professor::all()->each(function ($professor) {
            $semesters = Semester::inRandomOrder()->take(2)->get();
            $professor->semesters()->saveMany($semesters);
        });
    }
}