Laravel validation not exists in table

I have two tables, first one is students and second one is lesson_student table. They have m2m relationship between students and lessons tables. I want to check, if student_id exists in students table and not exists in lesson_student table, insert it. I use exists to check in students table but i have no idea how to check it is not exists in lesson_student table.

$rules = [
   'student_id'    => 'required|exists:students,id|???? not_exists:lesson_student ????'
]

btw i am using laravel 5.6

via Chebli Mohamed