When you are young, your period is very often irregular and unpredictable, making it difficult to plan for. Girls are also starting to menstruate at younger ages--10% of girls have menarche before age 10, and more than half before age 12. When you need the supplies, you need the supplies, and it is a difficult thing for youngsters to plan ahead and be prepared for. Having the supplies readily available will help keep school attendance steady.
Look at it from a business perspective: at work, we keep a supply of menstrual products in the women's bathroom so people don't have to leave work due to a lack of having them available, just as we keep a supply of headache medicines and various first aid supplies for the same reason. We don't need a drop in productivity when the solution is so simple and cost-effective as compared to what the loss of productivity might cost.