Aw, Jenn, I'm sorry! And horseback riding lessons aren't silly!!! More importantly, wanting to pay for that yourself certainly isn't silly.
THe funny thing is, Jenn, I'm willing to bet you're far more mature and responsible than many of the University students they interviewed!
I'm glad you haven't been discouraged by this, and I'm glad you went even though they apparently intended to hire someone older to begin with.
When I was in high school MANY YEARS AGO, lol!, my school had a program where you could lump all the free periods together in the afternoon so that you got out of school early to have a job AND go to school full time. This program was only available to people in the 11th and 12th grades. In fact, they had a whole placement system, and worked with businesses in the local community who would specifically hire people from this program for part time work. It's too bad your school doesn't have something like this available for you! If ony I could remember the name of the organization that worked with the schools to run the program... If I come up with it, I'll PM you. Maybe you could work next year to get your school to have a program like that? I know you're in Canada, and I don't know how things are set up class-wise there.
When I was attending those years, school started at 8:15 and ended at 3:00 or 3:30. There were two "free periods" and lunch. To participate in the work-program, they lumped the two free period and lunch together, and we got out of school at 1:00, so we could get to our part time jobs by 1:30 or 2:00.