Unfortunately, Microsoft Office has to be the worst program out there for that job, in my honest opinion.
I've had this problem before and I've heard of other people who had it before.
My suggestion is to try some of the things that the other posts suggested. There are some that sound really good here, although I haven't tried them myself; and some that I'm a little dubious about, although it doesn't hurt to try (usually).
If they don't work, and for future reference, I've found that it's easier to just type it up in another format and then copy and paste into word when you're done. I recommend either of two formats:
Plain text
# It's easy to write. No learning required if you've used Microsoft Word.
# Just use notepad or any other text editor.
# Lightweight format. Keep multiple copies of your document.
# It gets a little harder to envision what the text means especially on really long assignments.
Notepad is a simple text editor. You don't have any fancy formatting, colouring, bolding, or the likes, but it's ideal for plain text. Just about the only flaw is that it only has one undo. So you can only go back once, and it's kind of hard to predict how many steps. But it does have a helpful and very noticeable warning each time that you exit if you hadn't saved right before.
Hypertext, particularly: HTML
# It's a bit harder to use. But you could learn the basics at http://htmldog.com/guides/htmlbeginner/ And go on from there.
# Just use notepad or any other text editor.
# Lightweight format. Keep multiple copies of your document.
# It's easy to envision what the text means. However, some of the more advanced features (e.g: tool-tips) might not be particularly helpful if you're going to print.
# Cascading stylesheets help add style to a page, and it particularly helps if you needed to do that.
Of course, you could print a Hypertext (after careful configuring of your web browser) without copying it to Word first, but I suggested that, so that you could do any necessary formatting more easily and print according to a specified standard. But think of HTML as a standardised, portable and universal document format, whereas Word is strictly proprietary (only belonging to a specific company or companies).
Plain text is probably the best out of the two, since it requires less work and you just need to reread and format where necessary after you copy it into Word.
Both have the advantage of "Just use notepad or any other text editor" which means that you don't have to do any unnecessary conversions between computers and the like. (Do not attempt to use Word to write either though, or Dreamweaver or other "specialized" programs for HTML. They add unnecessary tags that make your text bloated, and that's back to square one.) They also have an advantage of being a lightweight format.
Microsoft Office files are often bloated, the same document in information-rich XHTML+CSS and drastically simplified Microsoft Word format usually have the HTML one being smaller, since HTML has more actual content per byte. Office uses something similar to Mark-up and all multi-byte characters, as Microsoft Wordpad also does. And historically they have been known to write whole file loading algorithms into a Word file. This lightweight factor makes plain text and hypertext ideals for keeping multiple copies of the same file. It's also easier to just e-mail yourself the document due to the lightweight, and save the USB for the Word copy, keeping the plain (or hyper-) text as a backup in case. (Sorry that I'm a little sceptic about USBs, but I mostly trust them :-P)
That's what I suggest to try to avoid this problem from happening again. This is mostly just for that. It could be too late now to start with this. But I hope that the things that the others suggested help! Good luck! I wish you the best!