The tradition at the local high school where I live, which has a very small graduating class, is for the valedictorian to make mention of the 30 or so classmates, each by name, with a memory or anecdote. While charming, this is impractical for a larger class. Mentioning teachers and other staff members is a nice way to give credit and thanks where it is due, as well as to hold your audience’s attention. The biggest risk a speechmaker runs is boring the audience. Be very careful with jokes or funny stories at others’ expense.
I think that one of the messages that should be present in a valedictorian speech is that good, hard work is rewarded. It's great to joke around too but there must also be a very positive message associated with the speech.
In response to the 2nd post-you are only in high school so have fun with!
I like the idea in the post above about thanking those that helped your graduating class to succeed. Not only classy and appropriate but it's just the right thing to do. Thank your class as a whole while you're at it - tell them how much you've enjoyed being in school with them (even if for some of them, you didn't). You can also address those of your class who are going into the military services, or the veterans in the audience.
I think inside jokes are fine - I mean, this is your graduating class, it is a speech to them, and if they understand the jokes, then great! Have fun with it.
You do have an opportunity and a microphone though, so don't let it pass you by too lightly. Call on them to enjoy life. To travel and see the world. To take care of and appreciate the people who are close to them. Tell them that time is sacred, and they shouldn't waste any of it from here on out.
Any teacher who has several years of experience will probably tell you that after a while, every valedictorian speech pretty much sounds the same. The fact is, you cannot come up with an original topic. Everything has pretty much been done.
That said, my best advice is to be original, be personal, but don't leave the majority of your audience out. It is true - your class will enjoy a quick trip down memory lane (come on, everyone likes reminiscing and hearing stories about themselves), but avoid using too many inside jokes. Also, valedictorian speeches that take time to thank influential people (teachers, coaches, etc.) are always appropriate and usually very appreciated. Going through the last 12 years and giving the one "Lesson of the year" (has probably been done but) would be good.
Don't be too serious. Don't try to say anything profound - you are only in high school afterall, you have a lot of life to live - look toward the future.