Invites can be in the shape of the number 5, You can easily make your own, tracing a "5" shape on construction paper and cutting it out. Another idea is printing invites on plain sheets of paper, rolling them up and tying them with ribbon as you would a diploma. For favors, make small party bags with school supplies kids will need the following year, graduation caps for them to wear at the party, T-shirts with something like "Fifth-Grade Graduate" printed on them or even framed class photos for keepsakes.
Because fifth-graders are around the ages of 9 and 10, it's important to have a venue that's appropriate for their age. You could throw the party at the school the fifth-graders will switch to, so they can get a feel for their new school. Talk with the school to verify and select an appropriate time. Other venues can be at their current school; recreation spots such as a park, theme park, pool or skating ring; or a parent's home if it can accommodate the class. Ensure you have plenty of chaperons if going to a crowded spot.
Serve fun foods such as pizzas or plain subs that kids can add their own condiments to, or even have a sandwich bar and let kids make their own. Check with parents to ensure you accommodate any children with food allergies. A graduation cake is a must; for fifth-graders, make a cupcake for each one with a number 5 candle. They can each blow out their candles together to signify the end of their fifth-grade year.
A "5" theme -- such as hanging cutouts of the number 5 all over walls with streamers connecting them -- is one idea. A graduation banner is a must. You can find festive banners at party supply stores or make your own out of construction paper. Think of child-appropriate themes, such as a popular kids' show geared toward their age group. Another idea is to take a theme from something the kids learned during their fifth-grade year, such as a book they read or historical events they learned about. You could also have kids submit suggestions for a theme and let them vote on it.