This week at my HS I am doing a photo scavenger hunt. They are competing against all other 3rd years. working great so far.

I teach elementary school with 6 different homerooms in each grade (5 & 6).  I do a reward system where they can earn points as a class.  Each class they can earn one sticker, maybe 2 if they are exceptionally good.  I have a big chart on the wall in the English classroom where they can see how many points their homeroom has as well as the other homerooms.  On the chart there are different levels that they can reach with each level having a different prize.  Of course the higher they get, the better prize.  For example they have to get 10 stickers to reach the first level and then they get to choose their seat partner for the week.  Some of the higher levels include a games day or a movie day.  Even though they are not technically competing against each other they still want to be the homeroom with the most stickers or the class to get the prize first.  I am not sure if that will work for high school but it works well for me.  Also, on the board I have a little chart with an O at the top and X at the bottom.  Through out the class I will move a marker back and forth from the 0 to the x depending on their behaviour and participation.  At the end of class if it is on the O they get their sticker. 

I have a class competition for each grade level that is tied to class behavior. If they all behave and are on time, they get full points. If they misbehave they get less points. I add all the points up and the class with the most gets a pizza party at the end of the semester.

As rewards for high schoolers - I show them an episode of a tv show.  Since a movie would take up more than one class, a TV show is just the one class.  I showed an episode of Doctor Who after finals last year and my kids all loved it and they are always asking for it again this year so I've set that up as my reward for them - it's really quite effective.