School party ideas

Fruit outside on a hot day goes wilty and icky very fast, so if you do that plan to have a fresh platter to bring out.
An assortment of mini muffins sounds like what i would expect to see. You can also baked oatmeal with fruit in it, just bake in a large pan and cut into squares.
I would order from a bakery some mini croissants, those are always a crowd pleaser.
And yes, absolutely lots of iced water, coffee and tea on hand

Further to HalfPint's comments on what sort of occasion it is.
In a way, it's like a cocktail party only earlier in the day and likely without alcohol
But lots is people meeting, talking, walking around.
So go for small bites easy to eat and handle.
Also, because if the heat, take more than usual care with food safety.

Given the time frame, a mid-morning snack would be good since most people will have lunch at around noon. It's too late for breakfast and too early for brunch; and sounds like a brunch might be overkill for a welcome party (unless you're thinking of a brunch like party). I think a fruit and cheese platter with crackers/breadsticks. Some "carbs", like mini scones or homemade granola/energy bars, would be nice and relatively light. Round it off with strawberry lemonade, tea, and coffee. My cousin once had a self-serve iced coffee station for her baby shower. She had thermos carafes of coffee and milk, along with a large cooler of ice, and a vanilla syrup bottle with a pump dispenser. It was a big hit.