I was raised RC, 12 years of Catholic school, Baltimore Catechism and I was NEVER taught such a thing. Nor have I been taught that in the EO church.
You should have been. Mary's mediation is the reason the Holy Fathers dogmatized our need for recourse to her prayers and intercession.
"The Lord, the apostles and the prophets have taught us that we must venerate in the first place the Holy Mother of God, who is above all the heavenly powers ... If any one does not confess that the holy, ever virgin Mary, really and truly the Mother of God, is higher than all creatures visible and invisible, and does not implore, with a sincere faith, her intercession, given her powerful access to our God born of her, let him be anathema." (Second Council of Nicaea, Session 4)
"Omnes cum Petro ad Jesum per Mariam." You never heard this, or the briefer "Ad Jesum per Mariam" in 12 years of Catholic schooling? We have a feast on May 31, "Mary Mediatrix of All Graces". This is the teaching of the Fathers, both Latin and Greek and Syraic speaking. I prompted MarMema on this a couple of years ago regarding the doctrine being central to the though of St. Gregory Palamas, for example. Fr. Meyendorff's book introducing the thought of St. Gregory is very explicit on this point, as were his followers.
"No divine gifts can reach either angels or men, save through her mediation. As one cannot enjoy the light of a lamp....save through the medium of this lamp, so every movement towards God, every impulse towards good coming from him, is unrealizable save through the mediation of the Virgin. She does not cease to spread benefits on all creatures, not only on us men, but on the celestial, incorporeal ranks." (St. Gregory Palamas, Sermons on the Annunciation)
"She receives wholly the hidden grace of the Spirit and amply distributes it and shares it with others, thus manifesting it .... The Mother of him who through his unspeakable goodness willed to be called our brother is the dispenser and distributor of all the wondrous uncreated gifts of the divine Spirit, which make us Christ’s brother and co-heirs, not only because she is granting the gifts of her natural Son to his brothers in grace, but also because she is bestowing them on these as her own true sons, though not by ties of nature but of grace." (Theophanes of Nicaea, Sermons on the Most Holy Theotokos)
This was hardly new to them. St. Germanus of Constantinople, says seven centuries earlier:
"No one is saved except through you, O Theotokos; no one secured a gift of mercy, save through you....in you all peoples of the earth have obtained a blessing..." (Homily on the Dormition)
Pope Leo XIII, for but a single example, clearly expressed these truths many times, quoting several Fathers along the way.
"Wherefore We first of all give profound thanks to God, the Giver of all good things, and we shall continue as long as life lasts to cherish in mind and heart gratitude for each and every favour. And next, there comes to Our mind the sweet remembrance of the motherly protection of the august Queen of Heaven; and this memory likewise We shall cherish and preserve inviolate, ever thanking her and proclaiming her benefits. From her, as from an abundant spring, are derived the streams of heavenly graces. "In her hand are the treasures of the mercies of the Lord" (St. John-Damascene, Sermon I. on the Nativity of the blessed Virgin). "God wisheth her to be the beginning of all good things" (St. Irenaeus, Contra Valen., J. iii., cap. 33). In the love of this tender mother, which We have constantly striven to cherish and to grow in day by day, We confidently hope that We may end Our life." (Pope Leo XIII, Encyclical Diuturni Temporis Spatium, 1)
"The recourse we have to Mary in prayer follows upon the office she continuously fills by the side of the throne of God as Mediatrix of Divine grace; being by worthiness and by merit most acceptable to Him, and, therefore, surpassing in power all the angels and saints in Heaven. ... No man can meditate upon these without feeling a new awakening in his heart of confidence that he will certainly obtain through Mary the fulness of the mercies of God. And to this end vocal prayer chimes well with the Mysteries. First, as is meet and right, comes the Lord's Prayer, addressed to Our Father in Heaven: and having, with the elect petitions dictated by Our Divine Master, called upon the Father, from the throne of His Majesty we turn our prayerful voices to Mary. Thus is confirmed that law of merciful meditation of which We have spoken, and which St. Bernardine of Siena thus expresses: "Every grace granted to man has three degrees in order; for by God it is communicated to Christ, from Christ it passes to the Virgin, and from the Virgin it descends to us." And we, by the very form of the Rosary, do linger longest, and, as it were, by preference upon the last and lowest of these steps, repeating by decades the Angelic Salutation, so that with greater confidence we may thence attain to the higher degrees-that is, may rise, by means of Christ, to the Divine Father. For if thus we again and again greet Mary, it is precisely that our failing and defective prayers may be strengthened with the necessary confidence; as though we pledged her to pray for us, and as it were in our name, to God." (Pope Leo XIII, Encyclical Iucunda Semper Expectatione, 2 and 5)
"The Eternal Son of God, about to take upon Him our nature for the saving and ennobling of man, and about to consummate thus a mystical union between Himself and all mankind, did not accomplish His design without adding there the free consent of the elect Mother, who represented in some sort all human kind, according to the illustrious and just opinion of St. Thomas, who says that the Annunciation was effected with the consent of the Virgin standing in the place of humanity.(5) With equal truth may it be also affirmed that, by the will of God, Mary is the intermediary through whom is distributed unto us this immense treasure of mercies gathered by God, for mercy and truth were created by Jesus Christ.(6) Thus as no man goeth to the Father but by the Son, so no man goeth to Christ but by His Mother. How great are the goodness and mercy revealed in this design of God! What a correspondence with the frailty of man! We believe in the infinite goodness of the Most High, and we rejoice in it; we believe also in His justice and we fear it. We adore the beloved Saviour, lavish of His blood and of His life; we dread the inexorable Judge. Thus do those whose actions have disturbed their consciences need an intercessor mighty in favour with God, merciful enough not to reject the cause of the desperate, merciful enough to lift up again towards hope in the divine mercy the afflicted and the broken down. Mary is this glorious intermediary; she is the mighty Mother of the Almighty; but-what is still sweeter-she is gentle, extreme in tenderness, of a limitless loving-kindness. As such God gave her to us." (Pope Leo XIII, Encyclical Octobri Mense Adventante, 4)