One fee can be applied to multiple classes.
For example, "Maths" can be applied to "Maths F1" and "Maths F2". If F1 & F2 are not a must to show in the receipt, then one fee (Maths) will be enough. You do not need to create two fees in the system.
1. Create a fee named "Maths".
2. Input the pricing model.
3. Add courses those are suitable to use this fee.
4. When selecting Maths in sales, two supported classes can be enrolled.