The other part of the question: "We are junior high school classmates .... and we haven't seen each other for a long time ...." => 우린 중학교 때 같은 반 친구였는데 오랫동안 못 만나며 지냈어요.

소꿉친구(어릴 때 소꿉놀이를 하며 같이 놀던 동무): friends who are playing at housekeeping with you when you were young. 예) 두 사람은 한 마을에서 자란 소꿉친구이다. 죽마고우(대말을 타고 놀던 벗이라는 뜻으로, 어릴 때부터 같이 놀며 자란 벗): A buddy from my old stomping grounds. A dictionary definition - This is your old buddy who was playing with a bamboo horse together when you are young. and your friend who was raised in the same place and hanging out with you since you were young.

Those have the same meaning. 소꿉 is original Korean. 친구(親舊) is from Chinese character. Here is an example sentence. 두 사람은 한 마을에서 자란 소꿉친구이다. 죽마고우 (竹馬故友) is from China. 대말을 타고 놀던 벗이라는 뜻으로, 어릴 때부터 같이 놀며 자란 벗.