After my recent visit to Paris during the height of back to school shopping I would have to say, YES, students in France really do commonly use French rule paper. While shopping I observed parents and students busily filling their shopping baskets with loose sheets and notebooks filled with French rule, or Séyès paper.
I found the two most common sizes of paper were A4, or 8 1/4" x 11 3/4", and 6 5/8" x 8 5/8". The smaller size of paper was even available in loose sheets and there were nice compact size binders to hold these pages as well.
Clairefontaine paper was everywhere! Just look at these stacks of Clairefontaine French rule copies doubles perforees and loose sheets! The available quantity and variety of Clairefontaine paper was definitely enough to make this USA shopper jealous! I might just have to plan a return trip to France and bring an extra suitcase.