Read & Write (sage ability)
Read & Write is an amateur-status sage ability in the study of Instruction which is a rare gift in a medieval fantasy setting, awarded only to those persons who have reason to acquire the skill. Virtually every bard, cleric, mage or illusionist can read, because those vocations require reading as a critical part of their exertions. Most other character classes can't read at all, unless they happen to have been raised in a family where the children were taught to read as a matter of course. It should be taken as canon that those who can read can also write, and they were raised around books and are comfortable even with difficult to read texts.
Reading and writing can be taught by anybody whose literate. The time needed is but 50 hours of steady practice, which must be spread evenly over a time of not less than two months. Nor should any one-hour lesson occur more than one week before the previous lesson. At the end of the first 25 hours, the student makes a success check by rolling a d20 against both their intelligence and their wisdom. If either are successful, the student has achieved the first of two milestones, and can read simple texts. Following the second milestone, when the student has been tutored a total of 50 hours, the student will be able to write legibly.
If a check fails, then the student is "set back" 10 hours in their instruction. When this ten hours is made up, the student can try to roll success again. This process continues until the character learns to read and write, or gives up the effort.
Although this process does make the character literate eventually, we must recognise it will take six or more years for the character to become fully comfortable with texts that we'd rate as "high school" or "university" level.