So if they are willing to brain the teacher eye-to-eye to steal their gun why wouldn't they just bring their own gun and skip the uneeded variable of "can we or can't we steal the teacher's gun"? Your argument hinges on these theoretical students having murderous intent but not the resources or logic to procure a gun in any way except stealing one from an armed individual.
And even if your scenario did come to pass unless they were the only armed teacher in the entire school things would still be better off as there would be other armed teachers to engage an active shooter.