Teachworks supports a number of Billing Methods, including Package Billing. If you want to bill your clients upfront for a package of lessons/hours that can be used over an undefined period of time, the package billing method would be a good option and you can find more information here: Package Billing Method Overview.
The "Package Settings" section in the "Account Settings" page allows you to choose how you'd like to track packages and you can find more information about each setting below:
Package Units - Use this setting to track the package units by "Lesson" or by "Hour". If your lessons are usually longer or shorter than an hour and you'd like to ensure it counts as one lesson towards their package regardless of duration, tracking by "Lesson" would be a good option.
Count "Missed" lessons as used - When completing package lessons, only lessons marked as "Attended" will count as "Used" by default. However, if you want to include "Missed" lessons in this count, you can enable this option.
Track Packages by - If you'd like package balances to be reduced when any child student in a family is selected, you can set this setting to "Family". If you'd like to track packages for each student separately, you should select the "Student" option.
Packages available when Invoice is - If you'd like to make package balances available as soon as you've created and approved an invoice, choose the "Approved" option. If you'd like to ensure the invoice has been paid by your client before the package balance becomes available, you can set it to "Paid" instead.
For more information about the Package Billing method, you may want to view these resources: