Nynorsk is taught in all schools, at least in years 1-10, as it is required for all pupils to learn both forms. The number you cited, 15%, looks to me like it may be the percentage of pupils who have Nynorsk as their main written form in years 1-10, usually chosen by their parents when they started school. I grew up in an area where Nynorsk was the dominant form, with 2/3 of the pupils in Nynorsk classes, and that was what I used as well. Then, when I started high school, we were all told to pick whatever form we preferred and stick to it, and a lot of us switched to Bokmål at that point. Just about everywhere, in newspapers, magazines, books and advertising, Nynorsk was and is almost completely absent, so it was just what we where most used to.