The high school version seems like it is an over extension of the product which may hurt/confuse the brand image as you pointed out. The issue I see with the high school Edition is what happens when the High School kids go to college. Do they now graduate into the College addition?
Can�t blame facebook for wanting to grow. There are other ways to grow however. You can grow out (high school addition) or grow in. By grow in I mean grow the loyalty level of your base. Facebook is in a better position to do this than Myspace. They can provide many more utilities and services to their existing base and get a bigger bang (use per user) out of each service than Mysapce does because Facebook has a more focused user base. Each additional service will be more widely adopted (use per user) relative with myspace. Thus Facebook can grow �in� easier, that is, create more loyal users. I define loyalty as the utility one receives from a service not necessary how much one uses the service. Users of Myspace may use the service often but they may not be extracting the same utility-usefulness from it as users of Facebook.
Which users will have a more difficult time living their lives without the service? Which service has fewer alternatives that users can turn to? I would argue Facebook for both.
Thus the higher price tag per user?
If Facebook still wants to grow �out� there are two ways they can do this, acquire new users or keep existing users(which is kinda like growing �in�), Like you said after college users drop pout of Facebook. This brings up the loyalty issue which leads to affinity. People in general have more affinity with their college than they do with their high school so I would think that an Alumni version would fit the Facebook model better than a high school version. They might already have one not sure I never used the service. This does not dilute the brand at all might even enhance it, also leads to other services that can be offered. It keeps existing users, loyalty level go up,and also may add more user so they grow �in� and �out� at the same time.
Facebook's potential is far more limited than MySpace's simply because it's got more turnover (churn, if you like) and the market size is much smaller (there are only like 20ish mil college students in the States). So the lifetime value of a user should be much smaller as should the total value of the user base.
Is no longer true, possibly justification for the larger price/user
My two cent