If you are being paid to improve their grades, warning them against password-guessing is a breach of contract.
If you are being paid to educate them, it's not really low-hanging fruit, although it is a very tasty one. Most people are more likely to learn the wrong lesson (beware of trick questions) than the right lesson.
There's nothing wrong with password-guessing -- as long as you know when you're doing it and don't mistake it for actual understanding. I'd have thought a tutor could teach students those skills without ruining their ability to grab extra marks in examinations by password-guessing.
In about eight months or so, I will be one of those (hopefully not starving) students. I'll be moving out to London to live with my aunt and uncle in a rather nice middle-class neighbourhood, while I study and work to prepare for university the following year. They know a lot of the parents around there and suggested that I begin teaching small groups of 8-to-12 year old children for maybe an hour or two regularly, and charge their parents/guardians a reasonable sum per child. I would be teaching them math and science in all likelihood. Apparently word will get around quickly if I'm competent so I might have a substantial number of customers within a few months.
My questions: