NO.
No one should write anything. It’s a C, not a D, not an F. Do you really want to explain mental health issues, when AO’s view them as possible red flags? It doesn’t matter what caused your mental health issue. Barring stays in psychiatric facilities, hospitals, extensive time out of school, and so forth, no, don’t mention it. And if one of those situations applies, your GC is primarily the one who should address it. A C doesn’t constitute urgency.
Okay, thank you.
I think I’ve given the wrong impression here. I’m no longer depressed and boarding school is a good fit for me. I’ve visited and my academic advisor from my scholarship believes that a GLADCHEMS school would be very good for me.
I’m no longer going to draw attention to the grades by sending an additional statement. Thanks to everyone who brought my attention to its negative effects on my application.
You are no longer depressed, which is awesome. So why tell a boarding school about it?
A C does not require an explanation. You want them to ignore it. Good luck.
Thank you so much for your advice
Can I PM you something? Its related to this topic but in my situation its a bit different.
Of course!
Wow, this thread really blew up. From reading the responses, I take back my first piece of advice mostly. Listen to the other more experienced people 
Thanks for the advice regardless <3
Same! Listen to more experienced people regarding this type of thing…
All potential boarding school applicants should be aware that the adults on this thread are trying to help you. As I tell my teens daily, you can be a very intelligent teenager but you still lack life experience. That is why every adult who has replied is advising OP to not call attention to her prior mental health issue, nor should she call attention to her C. Also, if OP believes boarding school is a good fit, I would recommend applying to a wider range of schools that have more favorable acceptance rates. Good luck to all.
I am curious- why is boarding school a good fit for you?
I’m in need of a rigorous curriculum and there aren’t many opportunities for that in my area, I have a solid plan for my future in college and prep school would help me achieve those goals, I thrive in a structured environment, I value independence, I’m ready to take on greater responsibility, and I’m looking for an immersive experience in education.
I ask this because boarding school is extremely demanding, and I am not even referring to the academics. You have described having a history of mental health issues which you say greatly affected your studies. How will you cope with the demands of boarding school? This is a rhetorical question. The reasons you desire to go to boarding school could certainly be fulfilled in a day school. I don’t mean to take the wind out of your sail but instead to give you something to think about.
Thanks, but my issues have again been resolved. It wasn’t just the mental health that took away from the academics, i also had a lot of family issues that took away from homework time. I’ve already consulted with my educational advisor about this. They were the one who initially recommended boarding school for me. I’m very motivated and i have no doubt that I can succeed in boarding school. Thank you
Even so… In addition to all the good advice other adults have shared here I suggest:
Read through The one thing I wish I knew before BS - #13 by lilyesh
Consider looking into what sort of support networks the schools you are applying to have. Better to have well established options and not need them. 
Yes, and to @meetme_atmidnight i would like to offer an easy update:
i did leave, and now i attend public school. i had the EXACT same situation where i was recommended BS by many. i think part of the reason why it didn’t work out was the fact that i definitely picked the wrong school, and also that i wasn’t ready to navigate life on my own. i also benefit from structure, and boarding school didn’t have that for me as much as public school did. now, to be fair, my public school is smaller than most boarding schools - and my teachers are my coaches and whatnot. so the community is a lot more present than most public schools. i think you need to think deeply about what schools you are applying to as well. for someone that needs structure, andover and exeter may not have that as much as a school like mercersberg or george, where structure and personalized attention are a lot greater.
I also struggled with mental health and “got over it.” but a new environment with new people often brings back new problems for those who weren’t truly over it.
I would also like to state that in my public school i’m in the top 10% (i was probably around the middle 50, maybe a little higher in BS), and my psat score rose over 100 pts from sophomore to junior year, because i was much better supported socially and academically.
im a senior now, and i would also like to state that my college lists look a lot different than they would’ve had i stayed. im applying to SLACs, and i would’ve been applying to my safeties as a reach had i stayed.
i go to a top 10 public school in new york, so my public school is academically rigorous. i just get less homework and more time to do in school activities. im usually at school from 7:45 to 5:30 every day, very often until 7 or 8 and sometimes later. often i go back and forth 3 times a day from school for social events, school events, and sports outings. i’m also at school at least once a weekend.
my public school also made it possible to get my diagnosis for autism, and get the supporting accommodations a lot of private schools don’t do or have.
if you think public school is the enemy, it is possible you have made it that way for yourself. i know i did.