Like the other tools included in Google Apps for Education (Calendar, Sites, etc.), Google Drive can be quickly accessed right from your email screen. Here's how:
Click the "Grid" icon at the top of the screen next to your email address (or your picture if you've added one)
Click the icon for "Drive"
A new browser tab or window will open displaying your Google Drive
If you're using a modern web browser like Google Chrome or Mozilla Firefox, you can easily switch back and forth between your Mail and your Drive just by clicking on the corresponding tabs at the top of the browser window