Sounds horrendous! Poor you. I don't think you're being too sensitive, I would be hurt/mad too. I would want some SERIOUS answers from the school PDQ, especially the SENCO. I'm not sure what the DSN was getting at re. your other child, hopefully/probably just an innocent (though unfortunately phrased) comment, rather than anything more sinister.
Im a parent of a type 1 child myself, but my little girl is only 8, so I am luckily in complete control. Scares me to think this might change during the terrible teens. I don't know how to advise you, other than stating the obvious: communication from school needs to be better! Re. the high readings, I hope the probe (CGM?) will reveal some answers. The skipping of the lunchtime insulin can't have helped though.
Hug from me