Replying to I feel a bit exploited...

Ok so here's the situation:

I got thrown into a small private school in SLC (About 70 kids) this year, I'm 18 and I'm a senior. This last week, my principal gives me a slip telling me I have two hours of detention, along with a 10$ fine for excessive tardies last quarter... I was late to class a total of SIX times. So i'm like "Wtf, whatever." Detention is scheduled bright and early Saturday morning at 8am, and i'm told to report to my counselor's house and she'll have work for me.

So this morning, I arrive to her giant fucking mansion at 8:02, with 4 other kids (who are all under 18) that were also assigned detention. It's POURING rain. Turns out she's planning on having some garden social la-dee-fuckin-da party tonight, and she needs things tidied up. Here's a recap of my two hour detention:
-I fix her sprinkler main, which froze and cracked over the winter.
-I pressure wash her deck and patio furniture
-She just had a new driveway installed, and the cement crew left a lot of their tools and supplies in her yard. I move loads of rebar, pvc pipe, 2x4's, and tools clear to the other side of her house. Oh yeah, and her new driveway is about as long as 2 basketball courts, and I'm not aloud to walk on it... I have to walk around.
All in the pouring rain.

The other kids do her gardening, rake and clean her yard, and other physical labor while she and her douchebag husband (who treated me like i'm some sort of illegal immagrant worker or something)  sit on her patio, and converse and laugh. 10 o'clock rolls around and i'm ready to get the fuck out of there, so I ask her If I can leave. She tells me I have to pay my 20$ fine for detention. I thought It was 10$. She says NOPE, 2 hours, 20$! I reluctantly give her the money and leave.

I'm not complaining about the work. I don't mind getting my hands dirty. My question is simply, IS THIS LEGAL?! I feel kind of exploited. Oh, and if I hadn't gone to the detention, I get more hours and fines added on, and the school holds my diploma until I serve the time.


Spark Notes: Saturday morning detention: I wake up at 7am and fix my counselor's sprinkler main, pressure wash her porch and patio furniture, and clean her yard for 2 hours, in the pouring rain. I also pay a 20$ fee on top of this. Is this shit legal?!