To generate invoices for future periods for manual invoices, go to Accounting->Invoices->Create an Invoice and complete the form. Select a date range in the future and check the box to include unreconciled lessons.
It's necessary to include unreconciled lessons because normally the invoice will only bill your students for lessons that have already been reconciled by their tutor.
Preview the invoice to make sure it is correct and then check the box to create the invoice and click to button to create the invoice.
For automatic invoicing make sure that you select the radio button to include future transactions. You will likely want to generate the invoices before the start of the invoicing period. Make sure to check the box to include unreconciled lessons so that the future lesson fees will be included.
Save the schedule and you're done.