Hey!
Coming from someone else with social anxiety (and a few other anxiety disorders to ice the cake nicely) I figured the first thing I mention should be that certain physical symptoms caused by anxiety can be pretty similar to those of IBS. That being said, I would highly reccomend telling your parent(s). I know it seems embarassing, but I promise that they won't think of it like that - they'll just want you to be healthy.
As far as being worried about school goes, in honesty, I don't think I have much advice. I'd say that telling teachers - if you're comfortable with it of course - that you're having stomach/bowel issues would be a good start, as they'll be likely to let you go to the bathroom when you need to.
I'd also suggest - if you haven't already - trying to get on the NHS waiting list for therapy. I've learned a lot from my time with therapy/psychotherapy and continue to. Also, I promise it's not as scary as you think. It's tough, but not scary. Good luck <3