When you apply for credit, your score dips a little, because they look into your history.
But when you have a CC card, never shut the account - that will lower your score. There will be a few cards you will never use again (those store cards that you get when you buy something for 0% financing) - but having just a few of them doesn't hurt your credit.
The other cards - linked to your accounts like AMEX, etc., are good if you use them regularly. They don't look at you spending $1000, then paying it off all in one go at the end of the month. I started with lunch every week, and paying it off every week. For my UK credit cards, I keep them alive by subscribing to things like the financial times etc. (that i can read online), then it automatically gets paid off with my UK current account that i can transfer money into from here. That way the computers know they accounts are still active.
so lunch or a starbucks once a week on your CC cards, paid off at the end of the week is great - because they look at your payment history. They don't care if you spend $10 per week and pay that off or $100,000 per week and pay that off. It is the point that you are paying back regularly.
Then the higher your credit score, the lower your APR is. My wife hated that when she could finally get a CC card for me through her account at a tiny credit union (they added me on!), that her APR jumped up high because i was fresh fish, after 3 years of being rejected from many CC companies (for having no history as opposed to bad history). As the years have gone on, that APR is dropping nicely.
School debt doesn't have too much of a bearing on your credit score. There are good loans and bad loans.