You're nearing the end of high school and don't plan on taking more private lessons - you should find a teacher who wants to help you accomplish what you want to accomplish during the short time you have left. If I may ask, why do you feel "need to take the exam no matter what"? Has it been your musical goal to finish the numbered stages of that curriculum? (If so, in my view that's a perfectly fine way to structure your musical development. The RCM syllabus is by no means poor in its breadth.) Personally, if I were a teacher with a student at your stage, I would invest my time broadening my student's appetite for different kinds of music, expanding their horizons. I don't know how well preparing for an exam curriculum complements that.
I remember that my teacher had a bit of a reaction against exams just because he'd had to deal with too many parents who were determined that child's progress be somehow measured. But it's different when the motivation comes from the student.
In any case, which "the easiest pieces" are depends a lot on what the strengths and shortcomings of your individual technique are. What have you worked on in the past few years, and where do you feel your strengths lie?