From a Google form to an automatic total per month

[Da Webmasta]

Hello,

Sorry for my bad English.
For a school project I would like to be able to do the following
I have a Google Form with a choice of 3 classes (K1A, K1B, K1C). When choosing a class you can choose the right student. In this example there are three children in a class. Finally, you can indicate how much a student must pay. I linked the form with a Google Spreadsheet. In the Spreadsheet I created 3 new tabs with the names of the classes (K1A, K1B, K1C). When I click on such a tab, I would like to see how much a student has to pay per month. (This should then come from the timestamp).

Perhaps a difficult task?

Google form link

Google spreadsheet link

Thanks in advance