The required attendance has been covered... As for the required volunteer work:
The Band needs a LOT of money to function... Most funding had been whittled away through the years (mainly the 80's), and the Band must remain self-sufficient. A lot of HS bands must pay for the fuel and drivers to use school buses to get to games and competitions (while the football team gets there on the county's budget). Uniforms must be dry-cleaned, lonaer instruments maintained (yes, all scdhools have them, even if you don't see it), sheet music must be purchased, and usually the colorguard hires a choreographer for their portion of the show.
In order for the Band to be a viable option for most students, they need to make a LOT of money. The concession stands at football games are usually run by the "Band Boosters" (parents of band members) so the proceeds of the sales come to the Band. Schools host competitons for more money. You will have fundraisers (ours was fruit sales) to make the rest. Whatever the Band does not make, YOU will have to pay for (Ex: we were invited to the Inaugural Parade in 1993 to represent the State... After funds were used up, each student needed about $600 for the buses, hotels, etc.). This is why parental volunteer time is MANDATORY for participation. If not, parents like yourself would duck the responsibility, and the band loses money (thus making it impossible for poorer families to participate).