I'm not sure how it is in your school but in mine, it was a lot of work especially during exams. There were no breaks and we were expected to operate at 200% efficiency. Most days, I was multitasking so much that I felt like I was two different people at once. My advice is to go for it if you can handle an intense workload.
The workload aside, I really enjoyed the challenge and would say it was worth it. It taught me proper time management -because it was like balancing work and study- as well as many other skills I use today. Also, talking about the skills you'll gain as a senior prefect will look good on your personal statement.