I don’t necessarily loose range, but the busier I’ve got the more I notice tiredness in my voice. So I’ve tried to switch up my teaching style so that I say and sing less and let the pupil do more (basically just filtering by asking myself “do I NEED to say this comment/do this demonstration?”). Also I’m often guilty of not warming up before starting teaching, which I need to work on.
Also I’d suggest don’t feel embarrassed when pupils can sing higher than you, even if you’re a super amazing soprano. You’re not there to sing better than the pupil, you’re there to get the most out of that pupil. Especially as a male teacher with mostly female students, I’ve had to learn to demonstrate things where I feel comfortable, share the principle and then let the student find it in their range. Hope that helps.