Constantly Changing Ideas
Your ideas and ambitions as far as school is concerned can be both formed and changed due a number of different things. Ultimately don't be afraid to change your ideas but if you have found something that oh have a passion for don't change at the first sign of trouble I would say only really change if you feel by changing you are giving your self better or more opportunities.
In my own personal experience I never thought of myself as a student who would change their ambitions and from quiet a young age I wanted to become a Graphic Designer and I had the whole plan figured out as I enjoyed art at GCSE, I would take it as an A-Level and go onto study Graphic design in Uni. However this plan all changed when I walked in on day one of AS Level Art where I met not the nicest of teachers and was told I could only really hope for a C grade, which would be no where near good enough for both my mind, my parents and especially not Uni so after a lot of thinking I decided Art as a qualification was just not for me so the next day, after a long talk with my mum over a hot chocolate, I talked to my head of sixth form and my art teacher about dropping the subject. In hind-sight my teacher telling me to expect a C was probably meant to be motivation for me to prove her wrong but now that I have been through my other three very essay heavy subjects I don't think I personally could of coped with the work load of Art as well as RE, English, and Sociology. And the extra time in school that dropping art gave me, allowed me to improve my grades in these three subjects.
Now when this happens you have two options in your personal life and two options in your academic life. As far as your personal life is concerned you can either walk away from that subject completely and have nothing more to do with it or you can keep it as a hobby or even a back up plan. I got quite luck with this as the subject I walked away from is quite practical I could have the option to study it in collage if I did need a back up plan but for now I keep my art as a down-time hobby and I draw for say my friends birthdays.
In your academic life when you drop a subject you can be lucky and pick up another subject that interests you if they have spaces left. However on the other hand you can simply leave your 'subject box' free, giving you more free lessons to concentrate on improving your grades for your remaining three subjects. My experience was a mixture of the two, I did at first ask my head of sixth form about switching to another subject such as Law or Media but they were all full so I just left my 'subject box' free and had the extra free lessons to work on improving my grades for the other subjects, and looking back this was the right decision.
Now I've changed completely to wanting to study an English degree and become a book editor but even now my plans might be changing due to an amazing media course I heard about on a Uni open day.
Overall My advise would be to stay open to change and don't be afraid to accept it but also when you know you've found some thing you are passionate about don't be scared to try hard to commit to your plan and try your hardest just always think about your options when you face trouble or the opportunity of change and, as cheesy as it sounds, always follow what you know you would be happy to do for the number of years you'd do that course or aim for a job you would be happy to do through out your life and follow what your heart ultimately wants with help from your head.
Hope this helped and that you liked it :)