Hello and thank you for contacting us.
You seem to be indicating two different things that happen at school. Firstly you indicate that your son is being bullied (if I understand correct) and secondly that he is not concentrating and focusing in class according to the teacher. These can be different things and you would need to explore them a bit more with the teachers. The information that you have I am afraid is not enough to enable us to have a clear picture of the issues that happen at school.
It would be best if you spoke with different teachers that know him and get a sense of his overall behaviour. If they believe that he cannot concentrate in class and you have also noticed that he cannot sustain focus for some time at home then he may need to be evaluated for ADHD. If however, he is being bullied, then some of his behaviours may be reactions to this. Again, you would need to talk to different teachers to ascertain whether he is being bullied and you would need to encourage your son to speak to you openly about any incidents at school. In all cases, keeping you son excluded or sitting on his own should really be consequences of a specific duration and following an incident than using it as a permanent measure.
Other things to consider are what does your son say about all this and how is he affected by this situaiton at school. If he is bullied then he should definitely not be the one punished and the school would need to take measures to protect him.
I hope this helps
Feel free to share any thoughts on this with me