Q. Is it considered plagiarism if a friend and I work on our essays together?

Well, it depends. If you both hand in essays with the same sentences or the same ideas it could be considered plagiarism.

It's great to work together, but do your best to keep your work separate. Similarly, when you edit or proofread your friend's writing be careful not to unintentionally re-create your own essay, and don't take anything from your friend's essay and add it to your own. 

Be aware of what constitutes plagiarism (see Types of Plagiarism) and how to protect yourself.