There's merit to the idea that renegotiation is bad because you don't want to look like someone who can't stick to his word or someone who is greedy. I'm going to disagree with that idea.
Something that many, many people seem to forget is that the employment process goes both ways. They aren't just evaluating you. You are also evaluating them. Never, ever forget that. You aren't Oliver Twist asking the master for "More" it's two equal parties coming to an agreement on a mutually beneficial business relationship.
You are a contractor. You say that the reason is because you're still in school. But the reality is that your being still in school is irrelevant to whether or not they could hire you. They can hire you right now if they chose, but instead they gave you a contract. Why? So they could evaluate you and decide (without the burden of benefits, etc) whether they would like you to be permanent. What isn't said (because it's not to their advantage to do so) is that you're also evaluating them. A contract ending is a lot simpler to explain than leaving a permanent job, whatever the reason.
They are evaluating your skills, attitude, aptitude, cultural fit and work ethic. As I said, you're evaluating them. You're evaluating your boss, the culture, the type of work your doing, a company's ethics and whether the compensation suggested at the outset is sufficient long term.
Do you see the term I used in the last sentence? Suggested. Even though you initially agreed to a certain wage, it was still a suggestion unless you signed a contract for the permanent job as well.
For some reason, people subconsciously give employers more rights than they have themselves. Would they hesitate to offer you less if their budget suddenly wouldn't allow it? Would they hire you anyway if you weren't a fit? You are an equal at this point.
The bottom line is that until the job becomes permanent, you're still not employed by them and you have no responsibility to keep working beyond your contract. You can absolutely renegotiate. The chances of them being distressed are pretty slim. If they don't like it, they'll probably just say "this is all we have budgeted and that's what the job pays" and it's up to you to decide whether to accept the job.
When they come back with "you agreed on 42,000" you can respond by saying that it was just a number off the top of your head when you were put on the spot. In that time, I have realized that I'm actually worth 50,000 and that's what I think I deserve"
Let me state this again though. If they've made a formal offer in writing for 42,000 and you accepted it, you can try to renegotiate but it is a bad idea. If you've only signed a contract for the summer, it's a standard contract-to-perm and you should feel free to explore your new expectations.