Did your Mom watch Jamie Oliver's Food Revolution? If so, a perfectly good reason to pack a lunch. It's really horrifying to see what school lunches consist of today. Back in the dark ages when I went to school, lunches were prepared in-house and didn't consist of the terrible already prepared food served today.
Can't You bring a lunch? I don't know if you are a girl or boy but just a "I'm trying to eat healthier." explanation may be enough. My son took a lunch almost everyday of high school until this year as he can now come home. He just laughed and said he can't stand the school food. Many times his friends would try to eat his lunch as it was better than what they have. He still went thru the line to get a drink and candy bar. Your close friends probably know why.
The old standby excuse for my boys , My mom- won't spend the money on bad food, is on a health kick, my mom is making me go on a diet, or whatever else you can blame on a parent. That's what we're here for.