High School Graduation / Proper Wording

I want to ask how the verbiage should be written for a afternoon gathering after a high school graduation? This graduation is for the first grandchild and we would like to invite everyone to an after party at a restaurant. The only problem is, we need them to purchase their own food. What we do not know is how to write this properly so there will be no suprises and it will not sound tacky. Please help.