It's normal to drift apart when you guys don't attend the same school. However, there are the weekends that you can get together, or week nights, you could study together. Email / text will also work. Just know that your bestie will find new friends, as you will. Nothing stays the same. Tell your bestie you would like to still hang out and see what works for the two of you. Good luck.
About half a year before my good friend move to another province.. I got a part time job after school. We drifted apart when i got the job because i had barely any time to hangout with him anymore. I didn't even know he moved until i looked on his Facebook and it said "lives in _____ Alberta" ... We don't really talk anymore (since that was 7 years ago) and i know he's got a new pal.
as for your friend.. She/he will make friends at the school she/he is going too. And you will find new friends too. But that doesn't mean both of you can't still be friends.. Make plans and hangout sometimes.
Asking a question on here Anonymously is a sign of insecurity, which is also a reason for your fear of losing your "Best" friend.
As you get older people and friendships evolve. Are you going to stay in close contact, like talking every single day? Most likely not. You aren't going to be around each other to make comments about things that happen in everyday life. Things like "Emily" wore the strangest outfit, or the "Josh" did the sweetest thing. Sorry, but that is life and most teens face it when they are college bound, or go into the workforce. You are just facing it earlier. That does not mean you can't still be for one another for the big things in life. I haven't been in high school in a very long time. I do keep in touch with my best friends from then, though. For the first time in a ling time we all not only live in the same country, but are actually in the same time zone!