If you are offering subscriptions in your school, you might have given the chance to your users to first "play around" for some days, by enjoying a Trial subscription period.
Extend Trial period
1. Ιn your Stripe dashboard, navigate to Payments → Subscriptions, and click on a specific customer.
2. Click on Update subscription under the Actions button.
3. On the customer's account page, scroll down on the Free trial days and customize the remaining days. 
4. Click on update subscription.
Change Billing Cycle
This process can also be used to change the billing cycle of a subscription. For example, if you wish to change the subscription renewal date from i.e, the 10th of a month to the 20th, you should first put the user in a Trial period. After the Trial period ends, the user will be a subscribed user, with the new billing date.
Prorate should be disabled to avoid giving credits to a user, after turning the user's subscription to Trial (when a customer’s plan changes during a billing cycle, a proration line item is applied on their next invoice. They receive a credit for the unused portion of the old plan and are billed for only the remainder of the month for the new plan). You can find more information here.