confusion in Db structure for fees and payment in school project laravel

in my school fee management system i have course category ,class ,teacher ,student,payment and fee models and when the class is created the course category and teachers are related to it and when a student is added the class is related in many to many relation , and also when the payment is created in each month the class is related with it in foreign key.....i already developed it but i confused in the way how to manage fee or invoice of each payment that is created in each month for the student i ....the student pay their monthly payment in cash for the admin and the admin registered as "payed " in the system .......i can show all the code if it is required ..........please help me ??