I like Duckie's sense of individuality - she sure dances to her own drummer!! So far we don't have to bring packaged items only in as long as we are sensitive if a child in the class has food allergies. For the most part the teachers keep things on hand for the kids who might react to something. Our teachers work hard to focus on not having too many sweets at parties and bdays, so a taco bar would be LOVED (by the kids, too).
If packaged or restaurant food is all that is allowed, ask about a platter or party size tacos. We have a chain called Taco Bueno that has them for a fairly reasonable price for parties, etc... Another option is to see if there are any deals for pizza at places that have "taco pizza". Here we can get pizza with salsa, taco meat, and cheddar - black olives are extra. Some even put lettuce and sour cream on the top. husband LOVES these and so do the kids. Pizza might be more cost effective if you can get them to cut smaller slices. Otherwise check out the frozen food section, providing that you can bring in a version that fits Duckie's allergies/sensitivities that is for her.
A baked potato bar would be easier to do if you have to do packaged food. See if you can make a deal with a restaurant owner to bake small size potatoes for you, provide new containers of margarine, sour cream, frozen broccoli to microwave at school, etc...You could just give each kid a potato and let them dish out the toppings.
What do you plan to serve the food on? If you have access to a Sam's or Costco, consider buying chubs - those paper basket like containers that ball parks serve nachos, fries, etc... in at some places. They are cheaper than plates and less likely to spill because they are more like bowls than plates. (They are great around the house for snacks, etc... also.) I dropped some off with snacks for thank you's class a couple of years ago and the teacher begged to know where I got them. I ended up donating a box because they were so handy for projects, etc... and did a better job of keeping the crumbs corralled so cleanup was easier. They don't go with a "theme" unless it is red and white but they make life a lot easier to clean up.