In addition to your student body and parent body networks, your school has other networks of contacts that should not be neglected when promoting your run.
One alternate network you can promote your campaign to is your alumni. Most schools keep an active database of alumni. Some schools have regular alumni communications such as newsletters or bulletins that you can get listed in. If your school does not already send out regular alumni communications, try to get the school to send out a letter or email to the alumni database promoting the Fun Run fundraising campaign. Request that alumni donate to someone they know who may currently be attending the school. You can even setup an "alumni page" specifically for alumni to donate even if they do not currently know anyone attending the school.
Throughout the year, your school purchases a variety of goods and services from an array of suppliers. The Fun Run fundraiser is a perfect opportunity to call or email your suppliers and to ask them for their support. Your suppliers and vendors appreciate your business, and will usually be happy to make a small donation, or even to sponsor your Fun Run fundraising campaign. Take a moment to think of all the suppliers your school uses; cafeteria and food, office supplies, gym equipment, computers and tech equipment, art and music, ... etc.