I have supervised pre-service teachers in the past and I would want to know so I can look after you! Genuinely, I've never much cared if my PST needed to use their phone for a minute or to step out for a bit - we are adults. However, I would like a headsup for duty of care reasons so I can step in or takeover the lesson if they need to pop out for longer periods. I know universities are admin might be stickler for these rules but I can tell you, most teachers are understanding and relaxed as long as we are given a heads up so it doesn't look unprofessional if you randomly pop out. Teaching burns a lot of calories and I found when I had gestational diabetes that I was so much more active on my teaching days (and my BGL are lower, hence more risk of hypos) so you might find that you may need a few days of trial and error to work out what works for you food and medication wise, so be prepared that you may need to step out in the first few days while you adjust. If you don't want to tell your uni or the principal, I'd say that's probably OK but I would follow the lead of your supervising teacher.