When I graduated 10th grade, I went to a different school, and many of my friends joined other schools to complete their higher secondary (grades 11 and 12).
I had known them for many years (although I don't know how my "many" compares with your "old friends" :D) and when I had to leave school, I had a choice regarding my friends: either maintain my old friendships through social media, frequent meet-ups, and parties, or to see and know less of them (we still stay in the same city and live pretty nearby, so I could still see them if I really wanted to and had time).
At our graduation party, I told them my decision, bold and clear. I decided not to continue the friendship. It went along these lines:
Guys, after leaving school, I'm afraid I'll be unable to continue to keep in close touch. I'll miss y'all...
We'll miss you too! KIT sometimes, okay?
Not a bad response, and to this day, I can still call any of them and chat for hours if I want and have the time...
You've got to let them know that you can't help it, and this is the only way you had left due to work, or personal reasons, but you'll really miss spending time with them. Don't lie to them, it's got to be genuine, or old friends will immediately know something's up.
I know that I had a different situation, as I was leaving school, and you don't have any similar excuse, but at one of your meet-ups, build up your resolve, and tell them what you've decided. Cold-shouldering is definitely not recommendable, as your friends will (9/10 times) be hurt and might not enjoy contacting you ever again.