HELP ME DECIDE... Please !

UPDATE : I’m aware what I did was super wrong, and I’m ashamed of and disappointed in myself . I’m not looking for criticism just some help. She’s a heavy drinker and goes off on even little stuff so you can imagine this is very difficult for me... she will go to the extreme punishment routes .

So Friday I posted a topic about something I did in June . I moved my butt a little on facetime.. in my underwear . I know it was a stupid thing to do ... I just felt like a baby compared to all of my friends . They were in the same grade but doing so much more than me .. Anyways, Friday I found out it was recorded and is now being sent to multiple people in my grade .. It’s incredibly horrifying and now I’m faced with a tough decision ... I’ve already deleted my account on snapchat and instagram (not just deleted the app, but the account itself) . My grandmother(guardian) works at my school as a lunch lady .. should I just confess to her what happened before she finds out st school, or is it less likely for her to find out ..? what should I say if I have to tell her ... Should I say it was in my underwear or just say I twerked on facetime ... PLEASE HELP ME . I go back to school tomorrow and must figure this out before it gets too late .