I stayed in touch with two.
One for 5yrs until she moved away and we lost touch. She never gave me her contact details for the new place and phoned a wrong number and left a message for me at a totally random house, which I obviously never received. She was chaotic and moved often, I called her old house when we lost touch but her family didn't know where she was either. We were best friends all our school days and I still miss her.
We found each other through Facebook after a decade, about 15yrs ago, she'd apparently been looking for me for a while but I wasn't on it. We messaged a bit at first and met up once, when I donated something to a cause she was collecting for, she was still in my area but we never hung out again or swapped numbers.
I moved away shortly after and when she stopped posting life updates on Facebook and just using her page to share lost keys/cats/phones and crimes from her town, I came off Facebook, since at that point she was the only reason I was still on it and I wasn't getting any updates. So now we're not in touch again. I Google her occasionally and it'll sometimes bring up some snippet of news about wherever she's currently working.
The other one, we were friends for over 20yrs. It was a weird friendship. She always included me if there was a group get together with other old school mates, even if she wasn't the one organising it. So in that way she made the effort.
She was a gossip though, our friendship was originally formed by mutual dislike of an incredibly annoying person at our school. We weren't bullies just vented to each other and also found we had other common interests so became friends.
I was going through a tough time in adulthood, she was gossiping about other friends to me including passing on some incredibly personal information about one's tough time and I didn't want her talking about me behind my back in that way. So I distanced from her by not providing information, which wasn't difficult because we'd both moved away from our hometown in our 30s and now live miles apart, but it left our conversations feeling awkward. She wasn't there for me, she'd always been a fairweather friend and it was something I accepted in her, but I wasn't willing to be the subject of gossip.
She took to only calling me when she was bored off work for extended periods anyway, which pissed me off because I felt used. We'd been so close, but I found out about her second pregnancy when she announced it on Facebook. There was a time I'd have been the first person she told, even before doing a test.
I called time on the friendship when she came for an event in the next town over, found time to also drive 40 miles to the coast for a day at the beach, but at no point had thought of catching up with me, her old friend who lived a 20min train ride from where she was staying and would have happily travelled there to meet her for an hour or something, if she was busy. Found out she'd been in my area when she happily told me next time we spoke. I decided I was done, next time I got a new phone I didn't give her the new number, we hadn't spoken in a year anyway. I rarely think of her now.
I never tried to keep contact with any of the others, although I was part of the wider friendship group we weren't close and they weren't in the habit of contacting me outside school.
I've no hard feelings with any of them. It's just life and fate really. People go in different directions. I don't think it's realistic to keep in touch, unless perhaps you all live in your hometown all your life and you're close.