This post on blogher.com gives good advice on figuring out how much ads on your site should cost.
Find out what blogs similar to yours are charging. Find other sites, even other school news sites, with similar traffic as yours. Here’s a comparison from compete.com for two good sites — The A-Blast and FHNtoday. Put in your own site URL and see how you compare. When you find a site with similar traffic, look for their rates posted on their site or email them and ask for a copy.
Go to BlogAds.com. Click on DIY Tool, then look for rates for a standard size ad, JPG format, for, oh, one month. For the search criteria, I tried “media” as a topic and got enough meaningful results to get a sense for what ads sell for. If you enter “school” as a keyword in the search bar you’ll get a few relevant examples, too. Consider traffic, of course — use compete.com to see how many visitors those sites get compared with yours.