The best defense here is to avoid any extreme contrasts in makeup coloring. Don't confuse evening looks with "special occasion". Now is not the time to try out the smoky eye, dramatic liners or deep, dark lip colors. As for weddings, you want a natural, soft glowing look that will keep the focus on you and how you look this school year, rather than that trendy shade of lipstick you're wearing. A natural, low maintenance look will best suit all the photo possibilities. (You might want to take the top you've decided to wear down to your favorite cosmetics counter for help selecting a shade that compliments you.)
Even if you don't normally have oily skin, remember that you'll end up in confined areas clustered with other people. Opt for natural fabrics like cotton since polyesters and nylon blends don't breathe and will trap in body heat not only for you, but everyone else around. Prepare to be hot and sweaty waiting in line for school pictures.
The key element is for everything to be light and airy -- from what you wear, to your shoes, makeup and hair. Be comfortable and you'll feel good and look better. Consider ditching your regular foundation, especially if it's a heavier consistency and try out (several weeks ahead of time, not the on picture day) something more sheer and light like tinted moisturizer, foundation stick applied only in certain areas or only a light layer of powder. Go for a natural sheer look, with matte tones. If you have very oily skin or tend to sweat, consider packing facial wipes and after the photos are over, take a few minutes to cleanse your face with these handy wipes and start over. This won't be a huge deal, especially if you pare down your makeup routine to just the essentials. You'll have a fresh, dewy look ready for the rest of the school day!