Tefillin Prepared by Children under Bar Mitzva

Several years ago, when I was 11, my (Orthodox) shul brought in a person who makes tefillin batim (boxes) and guided several friends and me to more or less make our own tefillin. Someone questioned me as to whether the tefillin are kosher because I was not yet bar mitzva. I would rather not ask my rabbi, who brought him in. Are my tefillin kosher?