Nobuta wo Produce. Exactly what you asked for, HS friend group formation. I loved it. Actually, I loved it so much that I also want suggestions of similar dramas, but I don't have any to add.
Brush Up Life is the best I've seen for female friendships. It's complicated and I found it hard to get into, but once it hit its stride it just got better and better.
Great list!! "My Boss, My Hero" is my comfort drama, I love Gakky's smile in this <3 "Pretty Guardian Sailor Moon" was a part of my childhood memories, Keiko was so stunning, ofc, she's always. "Last Friends" had such a good plot, and was unexpected.
Danchi no futari (it isn't exactly fitting with your description as this drama only focused on a friendship between 2 middle aged women lasted over 5 decades)