If you charge tax on service fees (class tuition), you'll need to set Tax Tuition and enrollment fees (excluding Web Registration) to Yes. When this is set to Yes, the Post Enrollment Fees box that opens after a student is enrolled includes a tax field and the tax is automatically calculated based on the Tax Rate entered in Edit Settings.
When using Post Tuition Fees (Class Based Billing only), the Services Taxable/Apply Tax field is preset to Yes. This field can be changed to No if needed.