You don't EVER want to get mad at her, never get angry, never yell or scold her, and certainly never ever hit or spank her, flick her beak, or anything like that...She's a baby Green Cheek, and that's how they are, it's not aggression, but she does need to learn about "Bite-Pressure"; If you search "Bite Pressure Training" you'll learn quite a bit about that...
However, if she's biting your ears then the best thing to do is not allow her to be on your shoulder until she learns that biting is a no-no...Birds respond to "Positive-Reinforcement" training, meaning you reward them when they do something good or do what you ask them to do, but never to "negative-reinforcement" or punishment...If your Green Cheek bites your eat when she's on your shoulder or head, you need to immediately just tell her "No Bites", not yelling but just tell her, and put her down. Being on your shoulder should be a reward that is earned...
Look-up "The Shunning Method" on here as well. It's the best way to get a "Velcro-Bird" like a Conure to stop biting. Basically whenever they bite you put them on the floor so they are lowest thing in the room, which takes away any dominance they feel (and they hate that), and you literally turn your back to them for 5 minutes and totally ignore them, pretending they aren't in the room, which they hate even more...They usually get the idea pretty quickly, that if they want to held or be on your shoulder they have to earn it and not bite...
***Important note on something you said...You mentioned that "your friends have struggled with her biting"...I highly suggest that you not allow too many people to handle her, in fact I wouldn't let anyone that doesn't live in your house or that isn't at your house on a very regular basis handle her at all...There are a lot of reasons for this, but the main one being that they are "flock" animals, and your bird's "flock" is you and whomever else lives in your home...Passing her around to friends or allowing friends to handle her that are not a part of her "flock" and that she doesn't know is a very bad idea, as birds are comfortable within their flock with the people and other birds/animals they know, and that's it...If she bites one of your friends that she doesn't know, that really is asking for continual issues...Plus, if your friends aren't bird owners themselves they may handle her the wrong way, or too rough, or touch her in a way that they shouldn't....Plus, if she's biting your friends enough that "they struggle with it", that's because she's not at all comfortable being touched by them, and rightfully so. They are not her flock, and if you allow your friends to come over and make her uncomfortable and then she bites them, that's not her fault at all, it's your fault for allowing them to touch her...I don't mean that in a rude way, it's just how it works with parrots...They aren't dogs or cats or any other type of "pet", and they typically do not EVER become comfortable with strangers or non-flock members that they don't see every single day handling or touching them...So that isn't something that you're going to change or that will ever change..I have 4 of the sweetest, nicest, most gentle parrots in the world, but that's with me and maybe my mom because she's over here a lot. And that's it. And even my mom is questionable, because she isn't a part of their "flock" and they aren't comfortable with her...So you need to make a rule that no one else but you and whomever else is in her "flock" that lives with her touches or handles her, because it's going to be impossible to teach her to stop biting you if you allow others outside of your house to touch and handle her, because she's NEVER going to be comfortable with that...Again, she's not a dog, and it just doesn't work that way...