This is hypothetical. I have no problem with showing my parents my report. I go to a private school, and a parent/guardian is usually required to sign a report as proof that they have read it. However, once I turn 18, surely if I rather them not knowing about my studies, they shouldn't know. Is it not personal information? An employer wouldn't send an employee's progress report to their parents. Are over 18s not in charge of their own progress. Let's say in another hypothetical scenario a 20 year old who lives on his own returns to school to finish sixth form. It might be the school's internal rules that parents must sign reports (pretty sure this isn't in any code of conduct private schools make you sign) but would they make the 20 year old who no longer lives with his parents go find them to get it signed. No. What's the difference with an 18 year old?
Another similarly themed question: private schools may require boys to have hair length not to be what they consider "long". I know at my school (from personal experience) that we are told to cut our hair if it's too long, and that some are suspended if their hair is too short. Unlike growing a beard, which in some religions is important, there's no necessity to grow long hair in any religions (maybe Islam?, but these men often cover their heads). Does that mean your personal beliefs count for nothing. If you had a strong belief that long hair was spirtitually important, or if you felt long hair is a key part of masculinity, or that you growing long hair is a sign of your dedication to the devil or something :/ do these views not count for something? It may be that in our culture it is the norm for men to have short hair and the school wants people to look smart as defined by what's smart right now, but that doesn't mean that a slightly older culture you prefer is no less valid. In the 60s it was usual for school boys to have very long hair. Just because times have changed doesn't mean you have to keep up.