My child is in constant trouble at school but good at home. What to do.

In all honesty, OP, it's a battle. Constant disruption, rudeness, lack of respect, lack of self control, problems with teacher recruitment and retention and low budgets have led to this.
I would love a relaxed atmosphere in my classroom. However, it's often not achievable with so many students who are not prepared, nor have parental support for, learning.
As you will have noted from my previous posts, I am sympathetic to your situation and I do think your child needs support and help and effective diagnosis.
I knew it would turn into a bit of a schools bash, but please consider why schools have had to go down this route.
Best of luck.

Maybe it's time to consider a change of school if you are convinced nothing else is going to work. Some teens just do not respond to rigid rules and sanctions, and they get demoralised, as you describe. If you genuinely believe the school regime is going to result in him doing badly in his GCSEs then perhaps a move now might work - but there is no certainty. It is puzzling that your posts don't seem to mention his friends (sorry if I have missed something) but if he doesn't have a good secure friendship group that again might be a reason to consider a move. If he is bright then you probably are going to need to push as much as you can to ensure he gets the grades he is capable of. What might actually motivate him to be a bit more studious? Final points, what have subject teachers said at parents' evenings, and look at his school exercise books to see if you can gauge how much work he is doing in class, it might be revealing.

But he doesn’t "get on well with everyone", does he? It sounds as if he has a cheeky, confident persona that appeals to some and not to others (the teachers he dislikes). If he knows in advance that he will be in trouble in some lessons, could he really not just keep his head down and keep quiet in those lessons? Either he doesn’t see it as important enough and chooses not to, or he has a medical problem. Your comment about being contacted if he is quiet, which you presumably thought was sort of amusing, shows that you don’t even expect him to shut up and let others get on with their learning. Are you sure he isn’t just living down to your expectations?

And if phones are banned in school, why are you encouraging him to take his and text Mummy from the toilets, ffs? You actually admit that you like him doing it!

It would be wonderful if you could actually sit in on some of his lessons, to see what’s going on, but I don’t suppose the school would allow it. You could try showing him the different lives of youngsters who did well in GCSEs and those who didn’t.

If all else, eg meeting the SENCO, fails, you might need to move him to a different school - but he might well encounter the same difficulties there. Continual low-level disruption may not be as bad as bullying but it is incredibly irritating for teachers and impacts the learning of everyone else. You don’t see that problem at home because he is not put in the position of having to share an adult's attention with 25 others or do things he would rather not do.

@VikaOlson the op hasn’t said the school is draconian though and clearly the teachers have varying tolerances to disruption. It’s fairly easy for dc to not be late for a lesson because he phoned his mum. Surely he knew what the teacher would think and what the consequence would be. Who on earth wants to be a teacher with so many dc being like this? Why are they like this? No one used to phone home years ago? We didn’t even have a phone. How come there’s any need now?

Sorry, have just seen that he has a sister so scrap that. Also that he has had issues with eating so meds might not be a great idea as they can suppress appetite. But apart from that, do definitely look into the diagnosis as it should help the school be more understanding of his needs.

Is he an only child?
I have a very able DS with ADHD. School has been a challenge to put it mildly with lots of similarities to what you describe (constant low level trouble, nothing vindictive/serious, endless detentions for disruption in class, calling out, toe tapping, humming etc and forgetting equipment, huge variation in his response to different teachers). Diagnosis helped as (some) teachers adapted their teaching style, he went on meds - we also stressed the importance of good behaviour/boundaries as SEN isn’t an excuse. He did GCSEs in the summer and did v well, though there are still bumps along the way in sixth form.
I asked if yours is an only child as DS is sweetness and light at home unless competing with his siblings!

I agree with previous poster's who suggested SENCO involvement but also do you think he would benefit from private tutoring? I remember at school a lot of kids caused disruption due to embarrassment of not understanding the lessons. With 30 kids in each classes, teachers cannot help every child in the way they need. Inevitably some kids fall behind and cause disruption due to embarrassment. Private tutoring may help in subjects he enjoys but is behind on.

Poor kid. School is just not suitable for all children and of course I do also feel for the other children and teachers who are disrupted.

My son also goes to a school where they are sanctioned for looking around, slouching, not eye tracking the teacher and even he (no additional needs) says sometimes he puts so much effort into remembering and following all the behaviour acronyms that he forgets to actually listen to the lesson content 🙄

Imagine having to go somewhere every day where everything you do just gets negative feedback, whether you can help it or not. Your very being and personality is a problem.

My son was like this until aged 14 when I took him to a careers fair. I was at end of my tether nagging about school. He sulked around the careers fair until he noticed something he was interested in (airforce). He now has a goal to work towards. His school behaviour and study ethic just switched. Maybe we were lucky but something similar could be worth a try. He admits now he mucked around because he didn't see the point in school.

I can see this from both sides - I am a secondary school teacher and a mum of a child like this. I think the whole school set up for boys your son's age is all wrong- they are bored, demoralised and can't see the point. They need more practical subjects, movement and interesting content. It doesn't solve your problem but keep talking - emphasise that school is the means to get to the next step. Talk about jobs, careers, courses for later on. If it makes you feel better, lots of these boys change in year 10, they grow up a bit!

There seems to be a movement towards a much stricter environment at many state secondaries. Unfortunately this really doesn't suit some children. That's also a long journey to school what time does he have to get up in the morning? I would seriously be considering a school move in addition to the ADHD screening.

Of course people need their Mums.
My daughter at university in a different country calls me every day.
My mum has been dead for nearly 30 years, I would love to talk to her.

The question is why does he need her at 1:40 on a Tuesday afternoon when he should be in Maths class and he will see her that afternoon?

As I said upthread, there are many strategies that the school can put in place. Usually, they have a very positive impact. Just make sure that you're in a meeting with the SENDCO and the pastoral head

I get it- but if the school keep doing exactly the same thing, they will get exactly the same results. Which benefits no-one; you, your son, teachers or other children. They should be able to call on resources/experience/senco to come up with some sort of solution other than punishment. He deserves a decent education just like everyone else. Is it an option that doesn't take as many GCSE's to be able to focus on the ones he might need for the future?