In March I am having 31 high school students and 6 adults for breakfast at my house.

Breakfast casserole with a simple fruit salad and bread on the side. It's easy and you can make one casserole w meat and one veggie to satisfy everyone. That's my go to tailgate breakfast.

Maybe some eggs and Lazania

I think a strata (or two) would hit all the marks for teenagers, adults... and the cook: hearty, tasty, and easy since you put it all together the night before and it benefits from an overnight marrying of flavors. Consider making one with sausage and one with vegetables (and a bit more cheese?) to satisfy meat and veg eaters. Serve a fruit salad with yogurt sauce on the side, and some muffins, juice, tea, coffee. Recipes abound. Voila!

Along with all the suggestions above, good granola bars are filling. Lots of good recipes on this site. Just type "granola" in the Search box and you'll find several pages of them.

Thank you for the quiche idea it seems manageable and tasty. These are really big kids and they need to fill up that morning.

Make http://food52.com/recipes/14244-quinoa-and-kale-crustless-quiche. I make this for all my large gatherings, and it is always a hit with all age groups. You can make it a day early, it is great at room temperature, you can add bacon if you choose. It is not messy to serve.

Have finger food: a variety of min muffins, mini quiches ( with meat & vegetarian), pineapple chunks wrapped in bacon, fruit pieces with a yogurt-based dip, a variety of cheeses, juices or fruit punches, coffee, tea & water. If you think there will be some really hungry kids, you could add deli-style meats, cheeses, mustards, pickles & breads for those who want to make a sandwich.