Hey! I'm kinda new here but I have social anxiety too (although it's not as bad as it used to be). What I usually do when I have to do something in front of people is detach myself from the feelings. It's easier said than done, I know, but try not to think. Whenever I feel myself getting a panic attack, I try to slow my breathing and count to 20 (or higher, if necessary), sometimes closing my eyes. I try to clear my mind and put all my thoughts to a screeching halt, telling myself that I'm more than my anxiety and people aren't judging me as harshly as I think. I don't know how much more severe your anxiety is than mine so this might not help, but I believe in you! You'll do awesome! (It took me a really long time to get the courage to press the post button but I've gotta confront my fears too. xD)

Then don't do it. Put your foot down. Unless it counts for a grade. Then do what ^ said.

First of all, I don't have a clue how the song goes, so if it means something to you (and ya 'cry every time'), put your foot down and say no.
HOWEVER, if you want to overcome/control/conquer your anxiety/stage fright, I would suggest the following (as a person who actually gets a 'thrill' from being in front of an audience, NOTE that I am now in my late teens and it all came with encouragement/practice) :)
1. BREATH, do exercises in a quiet room at home/school/work, possibly in front of a mirror and sing/talk to your self
2. Speak/sing in front of people, NOT A MASSIVE AUDIENCE, just people you are comfortable with (siblings, close friends)
3. Before performing, think of a different song in your head (preferably up beat) so that you can calm down, works every time.
4. Do a different song, potentially something that is up-beat/you are familiar with. This is so when you do perform, you are jumping around enjoying yourself and not concentrating on each face in the audience

I will tell you an super old secret: confidence
Just imagine everyone is wearing rainbow underwear and extremely vulnerable
I'm very bad at telling people get more confident tbh
If I could I would just let you borrow my confidence for a day cause man, I have never met anyone as confident and bossy as me
But break a leg, you would be just fine

Whatever you do I am sure you will do as best you can. p.s. let me know what you decide

Can we get an update??

Oh sorry yeah I ended up just beatboxing. Ended up getting a standing ovation from my classmates

Sorry for the anticlimactic ending...

Any public performance is a big deal when you have stage fright, glad it went well!!

The teacher let me beat box instead of sing

WOHHHHHHHHHHHH GOOD ON YA MATE!!!!!

Although she's now putting me in the musical and making one of the songs acapella *shy what have I done emoticon*