How Important is Length for a MG Novel?

I was hoping revising would shorten my novel, but so far, all I seem to be doing is making it longer. Some ways to shorten a novel:

1. Check to make sure every scene counts. Each scene should have a purpose in the story, maybe more than one purpose.

2. Cut lengthy descriptions. How many words are really needed to give the reader a picture of what's happening? Maybe a place or object can be described more concisely.

3. Read the story out loud, especially the dialogue. I find that dialogue can get wordy. Sometimes it ends up with words that don't do anything for the story, not even create a mood or establish a character's state of mind.

The problem is, even though I'm working on these things, I'm also adding scenes to fill in gaps or to create tension. So far, the adding is outweighing the cutting. At 57,000 words, this book is way longer than I want it to be for an upper MG novel. I know everyone says to write the novel and not worry about the length (and I didn't think about it when I was writing my first draft). But this is by far the longest book I've ever written and I'm not usually a wordy writer. Should I be worried? I'm hoping that when I get farther on in my revisions (now around Chapter 12) that there will be a lot more to cut.