I agree with investigating sensory issues. I don't agree with your mom friend whose attitude is that he just needs to get used to it and learn how to get along with others. Clearly that's not the issue as he interacts with the others at lunch and recess.
If he WANTS to HS and YOU WANT to do it, I say look into it.
It's not all Laura Ingals Wilder sitting around a candle in a log cabin with one book for six kids of different ages!
I've heard others report the social opportunities were better because the fit was better and there was more time for quality socializing. So, I wouldn't worry about that aspect.
I might try and make sure he is not sitting by the loud boys. Any possibility these boys are razzing him at lunch or recess? Could make him more sensitive to their antics in class?
I also don't think you need to worry that once you HS you can't go back. I have been told repeatedly that as the kids get older there are more opportunities for the school to meet their needs, particularly in High School.
Could you go talk to the teacher again? Or talk to the principal in a nice way? Could you express concern that school is visibly making him very upset? Maybe the social worker or school psychologist could observe what's going on in the classroom with your son.
That's what got the ball rolling for DD5 recently. Sometimes those professionals are knowlegeable with gifted issues and will be able to see what's going on for him if they spend some time in the classroom.
This opened up the opportunity to have a meeting and the principal and school psychologist were very nice and accomodating about communicating to the psyd. who tested the girls to come up with solutions. We don't know how it will go, but they are making an effort to try and meet the girls needs.
After many rounds with the teacher, it was a phone call to the social worker that finally got some results.
I don't know what else to tell you, just keep trying, you'll figure out a good plan eventually.
Hugs to you questions, don't give up.
Neato
