This is one of the oldest chestnuts around, a truly classic myth. A well-meaning head teacher decided children should wear safety goggles to play conkers. Subsequently some schools appear to have banned conkers on 'health & safety' grounds or made children wear goggles, or even padded gloves!
Realistically the risk from playing conkers is incredibly low and just not worth bothering about. If kids deliberately hit each other over the head with conkers, that's a discipline issue, not health and safety.