Follow these steps if the case is that your school is charging it by rate per subject.

Login to the portal using the accounts that can edit School WC settings, i.e. Administrators, Support, IT, etc.

Step 1.
Type School WC on the search bar above.

Step 2.
Scroll down and find "Per Unit Method of Charging" and click to box below to assign the rate "By Subject".

Click Save once done.

Step 3. Search for Subject Name List on the search-bar at the upper right corner.

Step 4. Select from the list the subject you will assign a rate.

or create a new subject if it does not exist.
Click new on the upper right corner of the page.

Step 5. Fill out the fields. Enter the rate on the Fixed Rate text-field.

Step 6.
Click Save once done.