Firstly, no matter how you are going to use the site, sign up.
If you use your school email account
to drop a line to teachers[at]instructables[dot]com
, and let them know that you are actually a teacher, you will be rewarded with a free Pro account
That will give you access to PDF files (potentially very useful in class) and all-steps display of projects (also useful if showing projects on-screen in class).