Karen, that's an excellent point. My husband and I have talked about how we would approach that issue when we had kids. It's just not a priority in schools to offer varied menus or to accommodate "special" diets. When I was in high school, there was a salad line where you could get salad or a baked potato, but I know that's not the case in most high schools.
In this situation, I think the best thing you can do is bring it up at local school board meetings. Another way you could raise awareness is if your kids are in extracurricular activities -- participate in potlucks, bring yummy vegetarian treats to bake sales or other fund raisers, and generally get the word out.
Here's an excellent article from the PCRM about vegetarian options in school lunch programs