In my experience applying to Pearson, the main contact person was very helpful answering questions. I remember writing a series of short answer and long essay questions, discussing things like my interests, passions, extracurricular activities, and what I saw as the biggest problem facing my region that year. Overall, it was clear that the selection committee wanted to find out who we were as people and how we saw the world. I found it to be a very reflective process, but it was a little stressful given that I really wanted to get in. After submitting my written application, we received an email letting us know if we were shortlisted or not. Those who were shortlisted were invited to an interview in Vancouver and were asked to fill out a family means testing. The interview was one day long, and we did activities such as ice breakers, preparing a mini One World performance to represent the province of BC, art collages, and a half hour personal interview with a panel of interviewers. I found the day to be very inspiring, but incredibly nerve racking. The team of alumni and board members who were facilitating the day and interviewing us were very supportive and welcoming, which made it a lot easier! My only frustrating memory was waiting longer than expected to hear the final decision after the interview was complete. My advice to someone who is applying would be to be as true to yourself as you possible can. Pearson is made up of real people with flaws, quirks, and insecurities.
Start researching UWC early and see if it is the right fit for you. Start thinking about the application process in Grade 10 and don't be afraid to reach out to current and past students for help. They are a wealth of information and can get you in touch with people who have similar interests, so that together you can see if any of the 17 United World Colleges is right for you.
Since you are only about 16 when you apply, it's a completely different experience from later on in life when you're judged on work experience, etc. It's certainly a combination of the things you've done, why you've done them, and how they fit in to your life and development. I think my sense of perspective and my eagerness to get out and learn and live things was a big part of my acceptance.