You are not being unfair. I never leave a class alone mid lesson to wander off to the toilet for five minutes or so and I am middle aged and have given birth to 5 kids. If my elderly bladder can make it through a 50 min lesson so can everyone else's. Medical exemptions are different. And I generally say to any teenage girl who is looking absolutely desperate 'is it an emergency?' in case they have suddenly started a period. I would not expect them to ask again for another 3 or 4 weeks, however. If the school has strict rules on uniform then I would certainly uphold them. Much easier to be firm about rules from the beginning. You're doing fine.