(I assume you are on the phone, or in person, it´s likely to say that).

In an eletronic schedule it would show if the specific class/lesson day and time is available or not. 

Plus, some systems will stablish a waiting time (say from 'now' to 48 hours). That is in practice you have to make your request in advance (Italki for instance). 

So, even if the teacher is a real person and teach in the old ways (say, at home - not online) to change date and/or time at the eleventh hour can be a headache to both, the student and the teacher.

https://support.italki.com/hc/en-us/articles/360019884934-Can-I-change-the-date-or-time-of-a-lesson-request-