您正在使用write (w)文件方法，该方法会用您传递的任何新数据覆盖您的文件。您需要使用append (a)文件方法，该方法每次都会追加到您的文件中。
The BSD fopen manpage定义档案方法如下：

The argument mode points to a string beginning with one of the following
 sequences (Additional characters may follow these sequences.):

 ``r''   Open text file for reading.  The stream is positioned at the
         beginning of the file.

 ``r+''  Open for reading and writing.  The stream is positioned at the
         beginning of the file.

 ``w''   Truncate file to zero length or create text file for writing.
         The stream is positioned at the beginning of the file.

 ``w+''  Open for reading and writing.  The file is created if it does not
         exist, otherwise it is truncated.  The stream is positioned at
         the beginning of the file.

 ``a''   Open for writing.  The file is created if it does not exist.  The
         stream is positioned at the end of the file.  Subsequent writes
         to the file will always end up at the then current end of file,
         irrespective of any intervening fseek(3) or similar.

 ``a+''  Open for reading and writing.  The file is created if it does not
         exist.  The stream is positioned at the end of the file.  Subse-
         quent writes to the file will always end up at the then current
         end of file, irrespective of any intervening fseek(3) or similar.

您也可以查看python的文档以获得更多信息：https://docs.python.org/3/library/functions.html#open

请注意

f = open(f"{fname}", 'w')

您正在使用w模式，该模式每次都会覆盖文件。请改用a+模式，该模式会附加到文件，并在文件尚不存在时创建该文件。

fname = str(input(">> Please Enter a file name, followed by .txt: "))
  f = open(f"{fname}","a+")
  for i in range(1, 4):
      print(f">> Enter record for student {i} in the format of [1. Name, 2. Mark]:")
      stdname = str(input(">>      Student Name: "))
      marks = str(input(">>      Mark for exam: "))
      f.write(stdname)
      f.write("\n")
      f.write(marks)
      f.write("\n")
  print("Student record writing completed!")
  f.close()
def main():
  writedata()
if __name__ == '__main__':
  main()

谢谢你们的帮助！这是我想出来的答案。