I know you made up you mind but you really should get school done asap... cost to go to school increases every year, also for books I would find out what I needed and then would buy them used on Amazon
for way less then the college bookstores.
I would go to school and then work after... I had 22 credits and 20-30 hours a week for work, I did manage to find some time to ride during school but we all have to make sacrifices some time in our lives. After two years I am just working, I have health insurance with my job, and I have some time to spend with the horses.
I would also work on Sundays, IMO you should be old enough to make some decisions on your own. If you are making it to church on Sat night then you should be able to have Sunday to work. Otherwise maybe work longer on Sat. And go to church on Sundays, if your parents are "mandating" you to do things they need to be flexible.