Yes, it is very much possible to hide your friend list of Facebook from those whom you don't want to see your other friends:
Just follow these simple steps:
1. Log on to your Facebook account
2. Go to your own profile page
3. Click on the Friends tab
4. On upper right, there will be an "manage" button with pen icon.
5. Click on that, you would get two options, where you should choose,"edit settings"
6. There you can choose the audience whom you allow to see your friend list,
7. To maintain complete privacy, you can choose "only me".
8. To categorize your friends list, you need to add them as "close friends", "friends" "family" "acquaintances" .
So if you categorize your school friends in one, and college friends in another, say, the first as friends, the second as close friends, then you can choose one of them to see the other or not to see the other.
Hope that works.