I can't help you with the prednisone part b/c I have never taken it but I would ask your doctor for a mild anti-anxiety med (like .25 of xanax). It is just enough to take the edge of, so you stop worrying about
the bathroom until you actually need one, but doesn't make you too tired or loopy. I find liquid diets really helpful too so you might want to try that leading up to the trip. Maybe by some canned meal drinks like Mix One or Ensure and have that for breakfast (if anything). If you are going with a big group will the tour be just your school? If so if you are on a tour bus and have to go, ask them to stop at the nearest bathroom. It will be embarassing but you will feel better. Also, with a bunch of kids they have to go all the time too so no one will know if you are stopping for you or if you are stopping for them.
Try to stay in the moment and recognize the difference between wanting to know there is a bathroom and actually needing to use it. I am still not good at that but I know I need to get there too. Also, even though the school requires chaperones for the trip, I still think you could get out of an activity or two if need be. Everyone gets sick. If a kid gets sick you can be the chaperone that stays behind w/ that kid. If there is a particular event you know will be stressful, after you get there, let them know you are going to miss that part. So if you were in Boston for example you could travel the city with the kids but not ride the Duck Boat with them, but be at the pier when they get back. I bet that would be ok. Know your limits and don't be afraid to ask for help.
Finally, are you married? Could your spouse come w/ you or a sibling to be a substitute chaperone if you can't make all the events? Good luck!
Oh, and you might be flaring from the anticipatory anxiety. I have a big trip coming up this weekend (big b/c it is a cross country flight w/ 3 take offs and I am a super fearful flier). I find I am "going" more but I think a lot of it is just nerves.
Oh, and don't forget immodium!
27 Year old married female law student (last year!!). Diagnosed w/ CD 4 years ago, IBS for over 10 years before that, which was probably the CD. I am sort of lactose intollerant too but can handle anything cultured and do well w/ lactose pills and lactaid. For crohns I am currently on Pentasa 4 pills/4x day and hysociamine prn. I also have bad acid reflux and have been on PPI's since age 13. I have been through prilosec, prevacid, and nexium. Currently I am on Protonix in the morning and Zantac at night. I take xanax prn for situational anxiety (aka no easy bathroom access).