The school may be able to help you by sending a letter to all students with a yearbook order form attached. Set your payment deadline for three weeks or more before you have to confirm the final order. This will give you time to give second (or third!) chances to buy.
The school may also be able to send emails for you (including ParentMail). If this is possible, see if you can have a weekly email sent out, with a slightly different message each time. Ask for a message to be posted on the school website as a reminder – ensure this is updated regularly too.
Arrange for tutors to help with collection of money too – this can be a big help.
To try to ensure that no-one is left to make up the shortfall on your yearbook order, be clear on deadlines for payment, stick to them and send out regular reminders (using one or more of the methods above). You could set up a yearbook payment scheme to allow people to pay in installments if they want to, but make sure that an up-to-date record is kept of who has paid.