I actually had this happen to me when I was like freaking 30 years old. It happened several times to me while taking up a mandatory course while collecting TRA benefits after getting terminated from a company that moved its work overseas.
For some odd reason this instructor was very strict with me, including with letting me go to the bathroom. She did not use ‘travel time’, but would ask me why I didn’t go between classes when I wanted to use the bathroom.
Basically you would say what I did: I did not have to go to the bathroom before, and I have to go very badly. Unless this question is satire, what else would you say!?