Talk to the teacher first. Find out how long the party should be. Ask about guidelines he may have for the party, food allergies, dietary restrictions and any other information he might want you to know. Settle on a theme for the party, one that will unify the food, games and crafts.
Get volunteers. A good rule of thumb is to plan for one adult for every six or seven children at this age. If there are 18 students in the class, two other volunteers should be sufficient. If there are 24 first-graders, you will want four grownups.
Decide on snacks and beverages. Small bottles of water are appropriate for kids this age and are sugar-free. Snacks should be somewhat healthy, but kid-friendly. They should not contain ingredients that any student is unable to eat. If you're not sure, ask the teacher.
Plan crafts that are simple. They should be easy enough for first-graders to finish in just a few minutes. Games should be simple and easy to learn in just a minute or two. Choose games that most of the children may already be familiar with, such as bingo.
Put together goodie bags. Fill small bags with little toys and trinkets to pass out during the party. Decorated pencils and character erasers make appropriate goodie bag filler. Limit the sweets and candy.
Create a checklist for the party and pack all the supplies up in a bin or bag at least one day ahead of time. Go over the list the morning of the party as a final check.