M10 is short for March 10th, the date that many independent schools release their admissions decisions.
For fun, check out “The Official M10 2021 Freakout Thread” in the Prep School Cafe section.
I have no idea what your chances are, but your info doesn’t include flagrant red flags.
There are plenty of good private schools that aren’t in the top 10; The GLADCHEMMS aren’t the be all and end all. Think about your learning style and what you want in a school, for example. Being able to show in your application and interview that you’ll be a good fit is very important.