Fucking hell.
If it's true (IF), i think the mom has reasons to be concerned.
I also think she has can raise her concern without revealing her power level is she just remark that it's a huge lie that there's a way to avoid going through puberty... there's not, unless you have a rare and debilitating hormonal disorder. You won't stay a child, you will grow whatever you do.
I think she could talk to the others teachers/headmistress by saying that it's weird and inappropriate thing to promise to a child ("even trans kids go through puberty eventually") , that puberty blockers have side-effects and that not everybody can even take them... do a little bit of research on several blockers (not just Lupron) so that she can argue.
She would need to be sneaky and have a good poker face, but I think she should also try and identify the Troon saying that to her child. And maybe gently but firmly told them to stay out of her child's business ...
The advice she has been given on ovarit are quite good