Employer tries forcing me to leave mid shift

So I work at a local McDonald’s for a temp job and my store is obsessed with labor percentages and sends people home often. They will literally come up to you while it's slammed and say "you need to go on break." I always say firmly "no, I'm not taking a break." Especially for a 4-6 hour shift. I need my hours. Usually most shift managers get the message, I'm here to work. Another thing they like to do is try to bully you into going home. I always say no. I've had 2 shift managers really try me on this and I remain my ground, "no, I'm staying my scheduled hours." I should add that l'm a great employee that stays busy, cleans, never calls off, always early, and get surveys from customers. Today I got TESTED!!!

I was scheduled 8am and before a "hi, good morning" the 19 year old shift manager says "oh hey, do you mind waiting until 8:30am to clock in." | said " I do mind, I'm working my scheduled hours." This is my first time working with her by the way. She was annoyed and proceeds to tell me "okay well since you're not needed, clean the lobby." "Ok, no problem." I sweep and mop. Then I get put on back cash. She comes up and says "ok, go take your break." | literally was there less than 40 minutes into my shift already. I said no and stood my ground. I then get put on drive through lanes with a headset. 20 minutes late she gets on headset to everyone and say, "xxxxx, go on your break." I respond again "I'm not taking a break." & she goes over headset "why??" Annoyed. Me, "I need my hours." 10 minutes later she approaches me and say "okay, go on break." I look at her and said "no, l'm not taking break. Where do you want me?" Her, " I really need you to go on break." Me, "I said I'm not taking a break and remain consistent on that. You cannot mandate me." She was so annoyed rolling her eyes. Me, "so which lane do you want me on?" Her, "whichever you want I guess"

20 minutes later, "can you clean the drains?" (Anyone that knows McDonald's McCafé service area drains, those things are NASTY. I said sure. She grabs me gloves and cleaner. A few other employees stared uncomfortably watching how she was treating me. I scrubbed that thing so clean. As I finish cleaning, I went to the back and was rinsing the drain cover. She approached AGAIN.

"OK when you're done rinsing that, you can go home." I was scheduled until 2 PM and this was around 10:30 AM. I politely responded while on my knees still from rinsing, " no l'm good, I'm staying the remainder of my shift." She turned around and said with the nasty attitude, " I'm not asking l'm telling." I Had a smirk on my face. I am very aware of what she was trying to do. I'm also all too familiar with my rights. I responded professionally, " I'm not required or at liberty to go home according to federal labor laws. I don't know how many more times I can make myself clear that I am staying the remainder of my shift." She then threatened that she was going to call the GM. I responded " OK please do." I did not see her for the remainder of my shift. The GM never came in or called to tell me I need to go home.

I want to write a letter of discrimination in writing and send it to the store for her behavior. do I have make a report to HR as well?

Also curious, have any of you dealt with this at an employer and am I in the wrong? I never worked with her, and she was not threatening other people because they had seniority. I honestly believe she thought I was stupid because I'm newer and unaware of my rights. I'm a single special-needs mom, working a temporary job rather than sitting on government assistance. Not that that matters, but it's wrong for the shift manager to assume all employees love the idea of leaving early and getting their hours cut.