Create a free Posterous blog and set it to private with a password.
A Posterous blog is simple to update - in your case, you would send an email with a file attachment(s). Within seconds or minutes you get an email reply from posterous with a permalink to your new post which contains the download file.
In your school newsletter you mention that link and the site password.
Advantages over dropbox (also a great solution):
- If you need to change the group password, there is only one to change and all downloads will be protected under the new one immediately.
- Parents only have to deal with one password (and they'll have the checkbox option of Remember password the first time they go to get a download).
- Inherently, you get a history attachments (it's a blog after all).
- You'll be familiar with a solid blogging platform if and when it's time to migrate technology used for the (mass email) school newsletter.