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...the Principal was very candid with us and admitted that if we were to keep him in immersion that there were no teachers in his school that would be able or willing to differentiate curriculum... What can I do to prevent this happening to other children?
Read all you can about advocacy, choosing a school, and finding the best possible academic "fit". Be willing to make changes as needed, many families re-evaluate each year. Talk with your child to see how they are perceiving their educational experience.

Encourage others to do these same things: read up on advocacy , assessing educational "fit, re-evaluating options frequently, and consulting with their children.

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poorly administered "assessments"
Development of reading skill has been discussed on several recent threads and may be found by searching the archives. Here is one post from one discussion thread about reading levels... a child may read (decode) at one level, read fluently at another (often lower) level, comprehend at another level.