The gempukku ceremony is a test for the young samurai to prove their expertise in the basic skills that their clans see necessary for their future roles. This mean that these tests vary greatly from clan to clan, and between bushi, shugenja and courtier.
Before the ceremony is conducted, however, it is safe to assume that every young shugenja is taught how to properly communicate with the kami, the same way a bushi is taught kenjutsu and courtiers learn etiquette and politics, along the array of skills needed to thrive in the court. Without those bases they would not be able to assimilate the most advanced knowledge in the form of schools techniques.
Pre-gempukku samurai should be treated as normal rank 1 characters in all senses, except that they have not their school rank 1 Technique, because they still has to prove themselves worthy of learning it. In the case of shugenja, this could also means that they have neither Affinity nor Deficiency that come with the school. Also, pre-gempukku characters have not earned the right to carry the symbols of their status as samurai (daisho for the bushi, and the wakizashi for everyone else). These are given to a samurai after the gempukku is completed.
As for spells, learning shugenja should have only the three basic spells: Sense, Commune and Summon. These spells are the building blocks from which every other spell are created, and represent the basic tools needed to develop their natural talent to communicate with the kami. All other spells should be reserved for those that had already proven themselves.
This should not be seen as an arbitrary restriction, however. Is only common sense to not leave potentially dangerous spells in hands of very inexperienced shugenjas. Also, many shugenja schools protect their knowledge, including spells, behind complex encryption systems, and do not share that knowledge without anybody that has not been fully inducted into the school.
Of course, this does not mean that the shugenja has no experience with spells other than the basic ones, they probably have. It would very possible that many gempukku tests include using one or more given non-basic spell, so practice with these spells would be allowed (under supervision of course). It is permanent access to the school scrolls what is restricted until they pass their gempukku.
A Final Note
Of course, this only apply to clan bushi, shugenja and courtier. True ronin do not necessarily have to follow these rules.