In the middle school that I teach at, I would say that 90% or more have smartphones. I'm less certain about the percentage in primary, but I would imagine it to be around 60-70% in grade 6, with maybe a 15% decrease per grade lower.
However, in both primary and in middle school, the students are required to hand their phones in to their homeroom teachers at the beginning of each school day.

That said, I do occasionally have them fetch their phones to use in class.
I use them for things like Kahoot (teacher-generated timed online quizzes where kids compete against each other for points), and looking up info for various projects that they'll later need to present.
The former doesn't require much monitoring, as it's pretty clear if and when students aren't participating, but the amount of attention required for the latter to make sure students are on task can be very substantial, so is best suited only for small groups.

   I've also used gamified language learning apps (duolingo) as part of my afterschool class curriculum. Basically I set weekly benchmarks for the number of units each student needs to finish, and the app's classroom settings allow me to track the progress of each student, which has worked out very well for me.
   
The kids love being able to use their phones in class, and even the phone app based homework seems to sit well with them, as it offers a lot of game style achievement rewards for pretty minimal time investment.
It does take a bit of effort on the teacher's part to integrate into the classes, and may meet with some resistance from co-teachers (especially older ones) who aren't comfortable with integrating technology into their teaching environments. Still, I feel that it has been worth the effort.