Most internship programs require a lot of extra paperwork and even some assignments, all while you're trying to learn as much as you can about the practical side of teaching. It can be overwhelming, to say the least.
The key is to keep yourself organized. Develop a simple system for keeping track of what's due when, and get it all done ahead of time so you won't have to stress at the last minute. Set aside an hour every evening for getting paperwork done and planning your next day; after you've finished, you have the rest of the evening free to relax.
Keep things in perspective, too. It's more important to have your lesson plans ready for the next day or week than it is to finish an assignment of your own. Do the lesson plans first so you'll always be prepared when you step through the classroom door. If your assignment is late, at least you didn't let your mentoring teacher or your students down.
I can't take the pressure! How can I keep from snapping?
At some point, you're going to get fed up with student teaching. Not only will you be sick of the whole situation, you'll be sure that teaching is the wrong career for you, that you hate everything about teaching, and that you're going to be a burned-out teacher before you even get your certification.
Relax. Everyone feels that way at some point. In fact, you can think of it as a good sign. The frustration you feel comes from realizing subconsciously that you're ready to strike out on your own as a teacher, but you're still stuck as a student teacher. The rest of the semester or year stretches out ahead of you endlessly--but the funny thing is, at the end of your internship you'll look back at it and wonder where the time went!
In the meantime, however, there are a number of things you can do to vent some steam. When you get home, call someone who will listen sympathetically--your mom, your best friend. Complain bitterly about everything you hate about teaching and just get it all out of your system. If you don't want to talk it out, try keeping a journal instead. Once you write down your problems, they don't seem quite so momentous.