Great to hear from you! Here are the answers to your questions (and for anyone else reading, hopefully you've already watched the film, which has led you to this forum -- otherwise, we're gonna spoil some of the film for you, so stop reading and watch the movie first!):
1. Why were they suspended?
Katie was suspended for stealing from the penny drive after Sammy threw Katie under the bus with Emily's help. Sammy actually gets expelled when she admits the truth of her role in stealing from the penny drive and covers for Katie -- you may remember from the beginning of the film that Sammy is already a troublemaker, so her punishment was worse than Katie's.
2. What was in the phone box when Sammy gave it to the principal? Was it something that implicated her?
The mobile phone is in the box. Sammy is admitting to Principal Martinez that she stole from the penny drive when she refers to the phone as "the proceeds from the penny drive". We don't watch the full explanation because we as the audience already know what Sammy has done. The next scene of Sammy sitting outside the principal's office with Angela arriving, and the door opening to reveal the police officer, Katie and her mom tells us most of what is about to happen next. Angela fills in some of the blanks in the following scene at the beginning of the car ride. Then in the final scene, we learn that Sammy got Martinez to clear Katie's record.
3. Where did the money go in the end?
That's a good question. If you listen closely to the newscaster audio when Hannah gets the mysterious text to turn on Channel 3 after Katie gets busted, we hear from the news that Principal Martinez promises that all proceeds will be donated as promised. Sammy doesn't steal all of the money. Just enough to buy the smartphone. We could assume that Principal Martinez tries to return the phone to get those stolen proceeds back for the penny drive as well. At the very end, Sammy realizes she could still do some good with the leftover gum she never got to sell. All of that money ends up at the women's shelter that was vital to Sammy's very existence.
4. I can't understand whether she was stealing and then was changed to be honest (and why she changed) or what really was the resolution.
Sammy was definitely stealing so she could buy her smartphone, feeling like the one kid who didn't have one. Sammy makes several bad choices during the course of the film, perhaps the worst being when she sets up Katie to take the fall for the penny drive to get even with Katie for sharing her Calculus secret (although, we sprinkle some visual details of Sammy rolling pennies earlier in the film, so maybe she was planning to sell out Katie all along. That's up to the audience to decide). When Sammy ultimately gets the smartphone, the one thing she really wants, she realizes it's a hollow victory -- she has no one to text. Katie may have been the one person who could have been her friend (again), and she sold Katie out. It's when Sammy realizes this that she starts to make decisions to try to redeem herself with not only Katie, but her mother Angela and her math teacher and mentor Ms. Dyer. Turning herself in to Principal Martinez to right the wrong she did to Katie just happens to be the hardest decision she has to make because she knows the consequences that she will face. The film leaves open the question of whether Sammy and Katie reconnect at the end, and I think it's up for audiences to decide where the two characters go from there.
Hope that helps clear things up. I also always recommend rewatching the film once you know the conclusion as you'll likely pick up things that you may not have noticed the first time around.