We really need to know what type of e-mail program you are using to receive and send mail. Whether you are using a web based e-mail program like Yahoo! or Windows Live, or if you are using a client based such as Outlook, Outlook Express, Endora, etc.
However, as you said you received the e-mail and opened the attachement without any problems I am assumming you are not using Outlook or Outlook Express, as both of those severely limit which file extensisions are allowed to be opened in them. File attachments such as .txt, .doc, and many other Microsoft Office Program extensisions are not allowed to be opened via Outlook or Outlook express due to how many people send malicious attachments.
So, this means you are either using a different client based e-mail program like Endora, or such or a web based program like Yahoo! or Windows Live Mail. OK, there is really no need to change the file name, as just clicking the save button or the save as if you are saving it for the first time as when you attach the file a copy is left in your My Documents when sending an attachment or in your My Music when sending a music attachment, or in My Video when sending a video attachement, etc.
It is simply really, open the e-mail in which you received the attachment. Click "reply", click "attach" and browse to your My Docments or if you want you can drag and drop the file to your Desktop and browse to your Desktop, click Open, then click Attach. Now click Send, and off it will go to the e-mail address your teacher sent this from. Make sure that the address in the header is the one in the Send To box, so it gets directed properly. There you are finished, good luck and have a great day.