While I'm not exactly familiar with where you are located, where I'm from FFA and Agriculture Education is part of the state curriculum. This means that FFA events and functions are excusable educational absences.
If you are still having trouble with this, I would recommend you contact someone at your State FFA Office and have them relay a message to your school corporation.
Also, take this time to educate your daughter's science teacher about FFA and Agriculture in general. Let them know that the National FFA Organization annual awards over $2 million to deserving high school seniors through 1,000 different scholarship applications.
Let them know that there are numerous career fields in Agriculture, many which do not involved production-agriculture. And that the average salary for these positions is drastically higher than that of a High School Teacher... (OK maybe not that!! ;D )
And don't be afraid to mention the premier leadership and personal growth that every past and present FFA member can testify to. Let them know about Career Development Events, Supervised Agricultural Experiences, and other programs/incentives like Proficiencies, National Essay Contests, the Washington Leadership Convention.
But most of all relate these to your daughter's life and show him how the FFA program will provide her with the experiences, skills, and other knowledge in order to be the best that she can be.
Try your best to educate and inform him respectfully. With decreased funding, many FFA Programs around the country are facing struggles and challenges. We need to educate others and let them know how crucial the FFA program is to today's young people.