My boss shows favoritism at work, what do i do?

I work at an elementary school for safekey and I observe a classroom at the same school before work. I told my boss I got done observing around 230 and I had to be at work at 300 so there was no point in going home for 30 minutes. She said it was fine and I could come to work early just not to clock in until I start working. I clocked in 10 minutes early on one of the days because I had been setting up for the kids to come in like putting tables out, since 230 so I was technically working for free. She got really mad that I clocked in early and yelled at me for doing it.

BUT my Co worker ( her friend) is supposed to be at work at 245 everyday but she arrives at 220 and clocks in at 230 everyday. My boss never says anything to her and allows her to do this. She shows favoritism all the time to her. I clock in one time early and she yells at me but my Co worker does it daily and she allows it. Should I mention this to my boss and ask her why my Co worker is allowed but not me? Or should I just sit back and say nothing because she is my boss?