I deal with sui thoughts and sh and certain things like isolation and/or blocking avenues of support from myself tends to make matters worse. I'd hope for things to get better not worse for you!

This sounds like a great time to ask therapist a lot of question about what she would do 'if'...
Maybe start the session saying you will not answer certain questions until you have a discussion about when and why she would report you, what her legal obligations are and what her take is on sui/sh and hospital stays.

If you are a minor, she probably has a legal obligation to report it if you just say you are having the thoughts and have a plan. This does not mean you both can't find ways to talk about it without reports tho.
She may not believe a hospital stay is the best thing for you at this point in time. Sounds like you are not unwilling to go to hospital if you are in serious danger? Sometimes (and I speak for myself here) we aren't always the best judges of when the line has been crossed and it's really good to have a therapist that can help one define what the line is and when it's been crossed even if one (i) don't always see it.

My pdoc knows I have thoughts and plans often. We've talked about it. We're both more concerned with the combination of certain mood symptoms and the thoughts/plans.

I do not know you. For all I know you could really benefit from a hospital stay and that is what will keep you safe right now. I think your therapist is the best judge of this.

I super encourage you to go to appointment and discuss the concerns you raise here about what it means for you to say certain things. For me, it was really helpful to have this discussion with my pdoc. He even discussed some of his perspective on sui thoughts and I was really impressed and reassured by what he said.

Good Luck!