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what do they do if the child already knows their numbers and can do simple addition/subtraction?

In our experience - force him to count dots with the other kids, recommend he get testing and medication for ADHD as he can't seem to focus in math (the other kids don't have problems counting dots you see), laugh at you when you say he already knows how to do this - because you are JUST the parent (and NO, despite the evidence, your child does not need more challenging material), then repeatedly tell him in class to not work ahead, send home notes to the parents telling us not to allow him to work ahead and to stop being so pushy, etc. When you provide supplemental material at the school yourself, they hide the folder or place it in an area he can't get to himself. Once he does manage to get it, humiliate him if there are ANY questions and tell him he is not smart enough to work said actually challenge material.
ITA. May I add: Some may put a note in the child's file to vaccinate future teachers against parental advocacy.

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Hmm, apparently I am still bitter.
Or not. The negativity toward gifted kiddos is prevalent and widespread. If parents who have been through it do not alert others, each family may feel they are going through it alone, rather than possibly bringing about change by raising awareness.

The methods of undermining utilized by some teachers/schools/districts may resemble "relational aggression" (girl bullying) or Zersetzung. This may include elements of destroying self-confidence, social isolation, damaging reputation, organizing failures in their work (in this context, possibly explained as a means of introducing "challenge"), inspiring self-loathing.

While parents help their children move forward in other learning environments, the effects of the undermining are chilling, make a lasting impression, and may cause lingering mistrust and fear.

For parents who may be new to this: There are many resources for advocacy information and meeting prep. There are success stories. If the parents follow best practices and the efforts are not successful in their learning environment, the consolation is that the evidence-based, documented, unemotional approach leaves little negative to be said about the parents, to be placed in a student's file.