administraCursos/database/seeds/ProfessorSectionSeeder.php

<?php

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

class ProfessorSectionSeeder extends Seeder
{
    /**
     * Run the database seeds.
     *
     * @return void
     */
    public function run()
    {
        Professor::all()->each(function ($professor) {
            $sections = Section::inRandomOrder()->take(3)->get();
            $professor->sections()->saveMany($sections);
        });
    }
}