"Higher Still was supposed to be about equal opportunity in education, but those who have the money for private tutoring have a distinct advantage over those who don't," he believes.
One Glasgow tutor, a full-time modern languages teacher who chooses to remain anonymous, agrees that there are dangers of over-influencing students. "I work only towards the exam," she says. "In Standard grade, the writing paper is given out to pupils in January or February and it would be easy for a tutor to do it for the pupil. I steer clear of this because I feel uncomfortable with it.
"Similarly, I wouldn't tutor my own school pupils. I see enough of them as it is! More seriously, I don't want a private relationship with pupils I teach and I don't think it would be looked upon at all well."
She charges pound;19 an hour and believes she does make a difference in her one-to-one hour-long sessions in the pupils' homes, helping them to pass when otherwise they might not or helping them to achieve better grades.
"Private tuition probably does give pupils an unfair advantage, but you could say that about almost anything you pay for," she says.
Her students this year are all local authority school pupils, though in the past eight years she has been tutoring, the majority have been private school pupils.
"I go to their homes because it's more business-like. I enjoy doing it but the reason I started was monetary."
Another Glasgow tutor, who charges pound;20 an hour for teaching physics and maths and has more than 15 years' experience, says he has never been asked to tutor a private school pupil but has experience of teaching in a city local authority school where most of the senior pupils also had private tutors.
"I wouldn't say it was unfair, though it is inequitable, but there's no level playing field anyway.
"I did teach two girls whose family lived on a council estate and who struggled a bit to get the money. Both girls were failing Standard grades at the time. I took them through Standard grades and Highers and both got to university. You can have a big influence," he says.
He teaches in the pupils' homes, never his own. "It's inappropriate to teach in your own house," he says. "And you should never be left lone with a pupil, even in their house. There should always be another adult in the house."
He recalls one occasion when, with a family he knew well and who knew him well - he had tutored the elder daughter - he was left alone with his pupil who asked him, quite innocently, if he wanted to see her pierced navel.
"Definitely not! was the retort. You have to develop a relationship but there has to be fixed boundaries," he says.
Most of his pupils have been girls. "They tend to be more motivated and will ask their parents if they can have a tutor. Boys are more likely to be pressurised into it by parents."
An Edinburgh tutor, a retired English teacher who charges pound;20 an hour, also finds this is the case. "I've mostly had girls," she says. "They're more motivated and relate more easily."
She only began the private tutoring when she retired. "I never had the time or energy before," she admits. The lessons began through requests from friends and then recommendations from teachers at her old school.
"Most pupils come out of insecurity, often they're the kind of pupil who never puts up their hand in class or one who feels the teacher doesn't know them. Contact is usually made just after prelims - the time for panic!
"I always interview the parents and the pupil in my house - because I teach at home - so that they know the situation. I tell them it's not magic, it's hard work."
She believes that the one-to-one relationship can bring great benefits both in terms of grades and in terms of drawing adolescents out of themselves. She also sees the insecurity in parents because they don't know the Standard grade and Higher Still exam systems.
"I always respect what the school wants. For example, with Higher Still and the student's personal response to literature, I always avoid that. I tell the pupils I won't touch it."
Most of her pupils, who often see themselves failing, come to get "a good pass". Out of the 30 or so students she has had in the past 10 years, only about six have come in order to stretch to an A pass for university entrance.
"I've been pleasantly surprised by the results I've had over the years and it's very pleasing to know you've helped.
"For me it's a great asset. It keeps me alert, keeps me in touch with young people and how they're thinking. I enjoy it.
"Is it unfair? Yes. Private tutoring is unfair but if people need it and can afford it I don't see why not.
"I'm not a saint or anything but I've also tutored people who couldn't afford to pay at all, people I know."