When you say that in your country and in your school if students misbehaved the parents would be called in and there would be a meeting with the teachers and the parents -
This is discipline, it's just being outsourced to the parents.
Many teenagers hate for their parents to be called in to discuss their behaviour. It could be argued that this is a punishment in itself.
I don't know what country you are from. I was a teacher for a long time so I can tell you that U.K. schools do also do this - if a student is getting a lot of detentions the parents will be called in.
In the U.K. this can result in a number of things depending on the school and the parents:
So we had an attendance officer and various pastoral support people. If the parents were struggling to get up in the morning (maybe disabled, or drug addicts or whatever) we'd arrange for pastoral support to ring every morning to check on them.
In some cases we'd give the child a lift to school in the school minibus (so send someone round to collect them).
Is there no disability or drug addiction in your country or was it just not at your school?
If it turned out that the parents and kid were getting up on time but the kid wasn't making it to school on time then suggestions would be made to help. For example one very intellectually retarded father (single parent) had heard that at secondary school students should make their own way to and from school. We weren't his catchment school so he was expecting his son aged 11 to walk over an hour through a city he didn't know to get to school. His son was frequently late and often got lost.
We put school transport in place to help the kid.
In some cases the teens were late or not attending because they were selling drugs or being used as drugs mules (county lines). These students absolutely did not come into class upset as their punishment was missing the teachers explanation,
As a school we worked hard to bring them into school and keep them there as they were safer in school than on the streets. We also worked with the police and social services to improve the home situations they were living (or not living) in.
Does your country not have drug addiction amongst teens and gangs? Most countries do,
Detentions are there to help children work out what is expected behaviour and what they need to sharpen up on. In the U.K. there is more of an emphasis on parental and own responsibility for learning. You and your son need to be sitting down and figuring out solutions
The detentions are to flag to him and to you that you need to fix a problem.
They also flag to the school leadership team and if he keeps getting them they'll ask you in for a meeting to see if they need to support your parenting, just like in your country.