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As an ENGLISH majorette myself, I think it is. It has all the elements of a sport: competitive (I have a big nationals competition this weekend), physical endurance, coordination, practise and...
You definitely do not HAVE to be flexible to be a majorette, but it definitely does help. I am not the most flexible person (for a majorette), but I still twirl! :) and love it! but flexibily does...
A boy can most definitely be a majorette. Some of the best majorettes in the world are boys. No sports are specified to one gender. Anyone of any sex can do anything.