These things are always tough because you have the gluten-free, the lactose-intolerant, the diabetic and the vegan to deal with. So you need an assortment with the absolute assumption that not everyone will eat everything. So for 70 people, you don't need 70 muffins and 70 yogurts.
And you absolutely need to delegate to the rest of the PTA board and any other parents who will step up. Usually teacher appreciation is something people will volunteer for because it's a one-shot deal!
If this is a buffet with a set time vs. a "snack all day" deal, have TWO tables so there's no line! That's a key thing with buffets - the first person stops to serve herself, and the whole line waits. So let people walk down both sides of the table, or have two separate, identical tables.
I'd get a large flat foil pan (with the catering stuff at the discount store) and a bag of ice. Set the yogurts in there, organized by flavor. We do some large training breakfasts once a month, and we do a lot of vanilla yogurt in small plastic cups with some granola, oatmeal or grape nuts on top, plus a variety of fruit toppings. Someone dishes those up ahead of time, and then it's a quick "grab" going through the line rather than a "stop and put on the toppings" delay. If you want to save money, this allows you to buy the large yogurt containers and dish out small amounts. However, it's time consuming, so you weigh that against the cost. If the average yogurt cup is 6 ounces, and maybe 50 people maximum will eat it, you need 300 ounces, which is really 9 large 32 ounce containers if you're going to spoon it out. (With toppings, people eat less). We usually have two egg casseroles (9 x 12 pan) for 45 people - sometimes one is plain and another has a meat (e.g. ham).
I agree about egg casseroles, or quiches which are easy to do. You can do a plain spinach frittata (which is a quiche without the crust) which helps the gluten-free group. Our group does a crockpot full of breakfast sausages (usually the turkey for those who don't eat pork). Those are easy if you make sure the ones in the center thaw - especially for a morning program. So mix them around, or bake them in the oven and then you can keep them warm in the crockpot. Alternatively, if you have those chafing dishes that hold Sterno, you can bake the sausages in one of the large foil pans previously mentioned, and keep them warm. You can also bake in a smaller one, put an egg casserole in another small one, and set them both over hot water in a large one set into the chafing dish stand. (We have all that equipment from serving cross country team pasta dinners and so on - if you think your PTA will continue to do events, it's worth the small investment to buy these things and just re-use them every year.)
I'd get the smaller size muffins - about 50. If you're getting bagels, I'd only get 40, and pre slice them. It might be easier to get the small single-serve cream cheese things - again, the line slows down immensely for people serving out of the larger containers.
I'd make a large urn of coffee and a smaller one of decaf. You should be able to find urns to borrow, which is cheaper than getting Dunkin' Donuts to give you all those boxes of coffee. Just remember they take a while to "perk". I'd put the coffee and orange juice on a separate table because people can be getting that if there's a line at the food table, and vice versa.
Don't feel you need to put everything out at once. With 1-2 volunteers there, more stuff can be replaced as it gets eaten. So put out 18 muffins to start, then replace. Same with bagels.
It's not awful if you run out of one thing for those who come late. People expect that. It's okay if you have options.
Have a plan in place for the leftovers. The local food pantry will take unopened products and whole fruits. The local food rescue group (if you have one) collects from corporate functions and hotels, taking food to shelters. They'll take the fruit platters with cut fruit, the baked goods, and anything that's been kept over a flame or on ice if it's perishable. If you have a veterans shelter nearby, take the bagels and muffins and opened yogurt cartons there. Your local police and firefighters will enjoy a platter of goodies too. Cut the remaining egg casserole into squares and put on a paper plate for them (they'll have a microwave for individual heating up).
Good luck!