You have every right to pose this question and expect a meaningful answer. However, the answer might not be simple and it might even cause you to ask other questions.
Firstly it is easy to begin by explaining what a Mystery School is not. It is NOT a study group gathered together for the purposes of spiritual study. It is a however a school convened together for a purpose conceived on the Inner Planes by Minds not restricted to physical incarnation working in harmony with its own initiates. The School is therefore rooted in the Inner Planes but expressed in the manifest world. The traditional terminology often used to describe the difference between a study group and a Mystery School is, ‘contacted.’ However as there can be no such thing as a non-contacted Mystery School, the nomenclature needs no further qualification. Now my answer should raise questions about the nature of mind and the nature of a wider reality. These are rightful questions but only time and experience can reveal answers to you.
The Mystery School Currruculum is designed to gradually connect each of its student with the Minds embodied in, and working through the body of the School. Understandably this connection cannot be forged without due preparation and progession. The Tree of Life provides the map for this journey of awakening and its three stages outline the Apprenticeship of the Soul-Journey.
Firstly the Lesser Mysteries admit the student into the group mind and provide the inner experiences that develop sensitivity, refine the energy field, apply meditation, and begin to shift the focus of awareness beyond the personality and towards a new identity in the Higher Self. This stage of personal development is cathartic, healing, and transformative.
The Awakened Higher Self as represented by Tiphareth brings a transpersonal Identification so that it is possible to co-operate more closely with the Minds implicit within the School through the work of the Greater Mysteries.
Beyond this awaits the conection with the indwelling Divine Spark and immersion in and with the Divine. This lifelong journey cannot be conveyed by anything other than an Apprenticeship of the Soul.
Dion Fortune has a great deal to say about all of these stages and about the work of the Mystery School in the world today:
1. The training of a student of Occult Science falls into well marked stages. Each stage is, or should be a preparation for the one above it, and serious harm is done when students pass from stage to stage insufficiently prepared .
2. The work of an initiate and, consequently the task the aspirant has to undertake in order to prepare himself for that work cannot be fully realized unless it be understood in connection with the process of evolution of which it forms an integral and very vital part.
3. The pupil of the Adept is known in the language of the Mysteries as an apprentice.
You might like to take each of these sentences as seed-thoughts for your contemplation.