100-125 people is really not a bad number, no really, it's manageble. First and foremost it would help if you knew if the principal is expecting coffee and donuts or a sit down breakfast. If he wants a big breakfast, I would really start with the kichen staff. No really, if he is working out a budget (oh my fingers are crossed for ya!) who better to help ya plan than the "lunch lady"!!! Everyday they feed hundreds of kids for so much$$$!!
Scrambled eggs, freash fruit, sausage, french toast and OJ or coffee is a simple menu to start with and can be done "cheap". Even a nice continental breakfast (rolls, fruit, bevage) can be done to look nice but cost just your arm (you can keep the leg!)
I would see how much the Principal wants to add to your budget, or better yet let him know to do this you need $X amount and the PTO can put in this much cash to pull this event off. This where the "lunch lady" can be a HUGE asset as as far as quanities go. Much of what I mentioned can be purchased at "Sam's" for half of what the caters will charge ya.
As far as donations, are you close to any farmers markets? Talking to them ahead of time you maybe able to make some great deals!
Okay....before I get this planned for you
check your budget from the school...
Deep breath....it's like planning a wedding was training for cr_p like this!!!