Hello everyone! *Is new*

I've just become part of the first soprano section in my highschool chorus and we're singing the Barber arranged Agnus Dei, and I find myself having troubles hitting the high B flat on the 9th page (for reference if anyone has the mixed chorus arrangement). I was wondering if anyone had experience and advice on how to hit it correctly and without straining too much? It isn't painful and I'm keeping my soft pallet lifted, as well as trying to support my breath, but though the sound produced doesn't sound nasal, I'm feeling pressure more around my nose (eep!) and even when I hit the note and it sounds fine at first, my voice suddenly drops out and I'm just breathing out air. I'll be talking to my voice lesson teacher but since I'll have to be singing it again before I see her I was hoping someone could offer advice? Is it lack of breath support or more likely throat tenseness? How can I hit that high note clear enough to carry through and how long does it usually take for one to transition from one chorus section to another? Suddenly being first is a bit dizzying!