It sounds like this is probably a parent issue rather than something you are doing. Firmness is the key. Give short direct answers for everything when communicating via e-mail.

Communicate with adminstrators whenever e-mails seem to be crossing a line of professional respect... they might brush it off but keep them in the loop in writing.

"Students are expected to practice to meet course objectives. It is likely that without practicing students may not demonstrate growth on required playing tests".

"Information about renting an instrument was distributed at our parent meeting and can be found on our website. It is the responsibility of the student to have a working instrument for band class".