Don't worry about it, it's why I only have myself scheduled to certain days. And anyway, if there's no big tests coming up, people tend to ask for it less.

[which is to say, this week she's had no customers, but it's fine. technically she doesn't even need to offer the tutoring services for pocket cash, but by now she's invested in making sure a few of her regulars get past their exams.]

You don't have to feel bad about asking, ever. If I was too busy, I'd just say so.