WOW! I can say that if one of my students had formally withdrawn from the course and then said 'oh I was just having a bad day and have changed my mind. Can I come back please?' I'd be seriously unimpressed. Thinking about giving up is understandable. Talking to your mentor about how stressed you are feeling and how hard you are finding things is fine. Ringing in sick for a day, when actually just needing a rest is understandable (not that impressive, but it happens). Making the decision to withdraw entirely, telling everyone you have, leaving a school to have to rapidly plan and prepare to teach a class all week, and then changing your mind back again is really not. The teacher will have told her class that you weren't coming back and sorted herself out to teach this term. To have to undo that, especially the telling the children part, with no guarantee you won't have another bad day and leave them in the lurch is a very, very big ask. It is possible that the school will give you another chance. The most likely is that you start this school experience again somewhere else later as a resit type thing. But it is entirely possible that the university and school say too bad, you chose to withdraw. Good Luck and I hope you get what is the best outcome.