Casual workers being told they can only take 14 days “vacation” a year. By “vacation,” the boss means any days workers says they’re unavailable.

Canadian here. I couldn't find anything specific on this but it doesn't sound legal. All the workers are casual and their 4% vacation pay is paid out on each payroll. Recently, the boss declared that ANY unavailability is considered a vacation request and workers are only allowed 14 days worth a year. My daughter is a student and is stressing out how she'll be able to continue school if she can't work with her boss to plan shifts around her school hours. This sounds absurd to me since they're casual and all of them are scheduled far less than full-time. Is this even legal?