It seems like people only stick with their own groups and of they are familiar with each other, they tend to stick together. I think if you were more sociable and talked to them more, then they might see you wanting to be part of their group and might start sitting with you. Just a thought.
In school I had to be the one to sit with other kids and only sometimes did kids sit with me. My mother told me something interesting in junior high that if you don't ever sit with anyone, people think you are the one who doesn't want to be with them so they end up thinking you think you are better than them. So after that I started to sit with kids randomly.
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Son: Diagnosed w/anxiety and ADHD. Also academic delayed and ASD lv 1.
Daughter: NT, no diagnoses. Possibly OCD. Is very private about herself.