There are different ways to loose a friend.
You and your friend may get into a big fight and not become friends again. Or, your "friend" isn't out for the best for you, so you must break up. Or, your friend is bad influence.
When you loose a friend, you may be sad. Why? She was so nice...and now we're not friends, you might think. Don't dwell on it for too long. Be sad, and then go on with your life. There are tons of people in the world to meet, tons of people in the world to greet.
Now, when returning to school and seeing your friend, do not ignore or be mean to her. Be normal. You may have quick chat's like:
Your Friend: "Oh, hi there!"
You: "Have a good day."
Your Friend: "Thanks, you too!"
You won't be as close as you usually are, and it takes a while to get used to that. Like I said before, don't be sad for too long. It may start to affect your health. Do things you love to take your mind off of your loss.
If your sadness continues, tell an adult. Write about it, and let your anger out by ripping unused paper, or punching your pillow.
You and your friend just weren't meant to be, but you had a good run. Now move on! :)