I am not a teacher.
You can say "few" without "a" but this will change the meaning.
Consider these sentences:
I have a few friends/a few of my friends = I have some friends/some of my friends.
I have few friends/few of my friends = I don't have many friends/not many of my friends.
Thus, "Only a few" means "only some" while, "only few" means "only a small number". In this sort of phrase you would often use "only very few".
Student or Learner