Short answer

Use the getSheets() method from the Spreadsheet object of Google Apps Script

Explanation

Google Sheets doesn't have a built-in function to get the sheet name by its position but it's possible to extended it by using Google Apps Script. The following is an example of a custom function that returns the sheet name given a valid position, otherwise return a custom error message.

function getSheetNameByPosition(position){
  var ss = SpreadsheetApp.getActiveSpreadsheet();
  // var sheets = ss.getSheets();
  var result = '';
  try {
    return ss.getSheets()[position-1].getName();
  } catch(e) {
    throw new Error('The position value is not valid');
  }
}

References

Extending Google Sheets

There are many approaches. Here is one without a script. Say, the columns B and C are populated from the form, B has the name of a student, and C has the amount they paid. Then the query

=query("FormResponses!B:C", "select B, sum(C) group by B", 1)

will get the total amount spent by each student.

You can then subtract the query result from their bank balance. Say, the query output is in columns E:F of Sheet1. To get the amount spent by the student mentioned in some other cell H2, use

=iferror(vlookup(H2, E:F, 2, 0), 0)

which means: look for the name in H2 in the column E, and return the corresponding value from column F (2nd column of the searched range). If there is no match, 0 is returned as the student did not spend anything.