I've been to these things and I never expect the school reception to be actual food...I expect/plan to either go out with the family or have a bunch of our own food at the house.

My kids like to stop by the table and get sweets/cookies though. So I would get lots of dessert items.

Also, graduations are always during a hot time of year even if they're indoors, so have lots of waters or lemonade, that sort of thing. Even more if your ceremony is outdoors. Seriously, I'm usually just thirsty.

I agree with Costco and would do —
Pinwheel sandwiches on the tray
The salami wrapped cheese sticks
The sliced cheddar squares (so yummy)
Crackers
Big bags of baby carrots
Big tubs of mini tomatos
Big tub of mini mozarella halls
Big tubs od humus
Chicken taquitos if you have an oven, and guacamole
Something for vegetarians like samosas or bean taquitos
Some kind of big salad from them.

Dessert:
The mini cream puffs imported from Italy and I think they also have Cheesecake Factory mini things in the freezer section.

Costco is probably cheapest. I have found that Safeway's catering is decent and you get a lot for your money.

Cheese/salami/crackers
Veggies/hummus
Cookies/brownies

You don't need meal-type food for this.