Coworker (16F) has poor customer service-etiquette, how do I guide her to improving?

I (27F) work at a restaurant/ breakfast buffet at a campsite. It is a casual environment. Two people typically work the breakfast shift, and today I was paired with my coworker who is a 16F. It is her first job, and she is very hard-working, analytical, intelligent, and logical. However, her social etiquette towards customers/ guests of the campsite could be improved.

When it is busy, she very easily becomes stressed and moves with much haste in a way that is loud and creates some messes. I can tell that her tense energy rubs off the wrong way to customers.

Sometimes she does not make eye contact with customers when asking them for their money, or she will yell across the way to communicate with them.

She places things down onto counters very loudly and with haste in a way that startles myself and others. The thing that was most troublesome today was how she was carrying a bucket of clean but heavy dishes onto the counter space in front of the customer who was standing with their tray of breakfast, talking with me about coffee and payment. She reached over the customer’s arms and across the counter to place clean silverware in a organizer beside her without any “excuse me, please” or verbal acknowledgement to neither myself nor the customer. When she finished, she picked up the bucket and quickly moved behind the customer and bumped them, so their glass of milk got knocked and milk sloshed onto their tray and made a mess of the rest of their items. It was a very rushed and sudden interaction and the customer was sure to make a comment to me about my coworker being “rough.” My coworker didn’t recall this happening, it was like she was on autopilot without regard for others.

I mentioned the milk incident to my coworker and asked her to please say “excuse me” when you need more space to work and customers are in the way, and to be mindful of the way that we place things down so as to create a peaceful environment as these guests are relaxed, on vacation, and we are the first people they are interacting with on the campsite.

She is proactive in asking for me to give her tasks, but she also offers to do things “for” me in a way that seems submissive — I don’t like being in that position. Yet, she also tries to take over my tasks which i’m actively working on, which causes confusion.

I try to be tactful about the timing of when I make these suggestions/ requests and I never do it infront of the customer, but I feel that the importance of this etiquette is not getting across to her. I also feel that I don’t understand teenagers very well, their senses of humor and emotional maturity, etc. I’d like some advice on how to re-frame my thinking about her being a teenager in her first job, and also on how to make suggestions of guidance so that she can improve. I feel that the more I work with her the longest the list grows and I feel a resentment growing towards her, I don’t want that.

Edit: rephrased one sentence to flow better