sue,

Definitely go for authenticity, rather than trying to estimate school performance yourself. I expect he'd sniff out any doubtful scoring pretty quickly, which would tar the whole process.

If you think you could get teachers to chart this accurately and consistently--and not sabotage the process--you could explore extending this into school, with their cooperation. Otherwise, stick with home, where you know just how reliable your data is.

...pronounced like the long vowel and first letter of the alphabet...