The reason students from some prep schools aren’t interviewed – many prep schools are located in rural, remote locations (like Northfield Mount Hermon, Phillips Exeter), where there are hardly any alums and as many as dozens of seniors applying. Logistically, alumni interviews just couldn’t be arranged (in the pre-Skype era). At many of these schools, there is a direct contact between college admissions and HS guidance – there is a conversation where all the applicants are discussed in-depth. Some prep schools are just a different beast. Now, there are many prep schools where students do get alumni interviews – I’ve interviewed many students from prep schools.
And fogcity and Lily are wrong – it’s too late now to arrange an interview.