All Rune Magic comes from a deity and reflects some power of the deity or something gained by the deity or some deed performed by the deity. Thus, each spell has at least one HeroQuest or GodTime Deed associated with the spell. The way that Rune Magic is taught is by re-enacting those Quests, being taken through the Quest by a Priest or Acolyte, being taught what happened, what the spell means, what it does and how it should be used.
Each ritual would consist of the Acolyte taking the class through the deeds done, introducing the particpants, showing them what happened on the Quest, explaining the powers of the spell and the responsibilities carried with the spell. As this is a walk-through ritual, there is no penalty for failing the Quest as the Quest cannot be failed. Those who play opponents in the Quest do not receive penalties for being beaten, as it is not a full Quest. However, there is nothing, in theory, to stop a HeroQuestor appearing in the midst of such a ceremony and fighting the participants, although anyone who did this would be the target of huge reproisals from the damaged temple.
Each spell takes a whole day to learn. Much of this time is taken up with preparation and fasting, ritual washing, praying or whatever. The actual ritual probably only takes a few hours to perform. As a real world example, the Endowment rites in the Mormon Temples take two hours to perform and teach things to around 50 people at a time, but people normally spend all day there doing preparatory rituals. I cannot see any reason why Rune Magic Rituals would take much more time.
At a shrine, the acolyte knows the ritual of his spell inside out and can willingly perform the ritual when asked, especially when given a little gift.
At minor temples, the acolytes and Priest would know more rituals and would each probably teach a spell every day, with the same spell being taught on a particular day. This means that spell teaching for the common spells would be available once or twice a week. Rarer spells would probably be taught when demand arose, perhaps in special classes.
At Great Temples, acolytes may well be specialised spell teachers, knowing the rituals for teaching one or two spells and teaching the same spells day in and day out. As there would be many acolytes teaching spells, all commonly available spells would be available each day, with many rarer spells being available each week. Of course, special classes can always be arranged, provided the correct gifts are given.
These are sample rituals which give a flavour of the Rituals used to teach Rune Magic. If your campaign is that way inclined, you could use the rituals to show that Rune Magic is a special kind of magic, to teach new players about how the deities interact with the world and to give them a feel of how HeroQuests work. It could also be an introduction to HeroQuesting - the initiate begins by learning the spell, then goes on to a Practice Run HeroQuest in the outside world.
Orlanthi learn the Shield spell by performing the "Arming of Orlanth" ritual from King of Sartar. Here, the initiate pledges to uphold the world order. Heler, or his representative, then proceeds to dress Orlanth in ritual gear, saying things like "here is Turnspear which is a Shield for you", "here is Humakt who will slay your enemies", "here is your helmet, Spare Me" and so on. When he reaches the item representing the spell sacrificed for, in this case the mailcoat "Turnspear", he will recite its properties further. Finally, the initiate will pledge to use the spell to uphold the Grand Order and to fight Chaos. This ritual can also be used to learn Great Parry, Lightning and Thunderbolt, as all these were borne by Orlanth in the Arming of Orlanth.
A spell such as Lightning Spear may be gained in one of several ways, depending on where you come from. You could use the Arming of Orlanth, as above. This is used where the myth of Orlanth gaining the Lightning Spear is not known, but it is known that he used it. You could also use the Hill of Gold quest from Orlanth's viewpoint. In an Elmali tribe, Orlanth would meet Elmal, recognise his kinship and offer help and friendship in return for the Lightning Spear. In a Yelmalio area, Orlanth would meet Yelmalio and would be challenged by him, in the struggle he would gain the Lightning Spear.
The Earthpower spell would be learned by Storm Bull worshippers in a ritual where Storm Bull is fighting the Devil, is wounded and thrown, dying, to the ground. Eiritha then gives him the Earthpower and allows him to spring up again to continue the fight. Storm Bull worshippers are taught that the spell is only used in emergencies, that it is not cast but goes off automatically and that unless the initiate always fights Chaos, he will not be worthy to use the spell.
Eiritha worshippers would learn the Earthpower spell in the same ritual, but they would see it from another angle. The husband of Eiritha is fighting the Devil and falls, mortally wounded to the ground. Eiritha comes to her husband's aid and uses the Earthpower spell to aid him and allow him to stay alive. They are taught that the spell can be cast on others to give them aid, that as long as the initiate continues to succour and aid their families they can be worthy to use the spell.
You can see the pattern here. The fun thing to do is to identify rituals for each of the main spells - Sandals of Darkness - Orlanth steals them from Kyger Litor, Scarf of Mist - Orlanth befriends Hurya, Rain - Orlanth frees Heler from Aroka etc.