Lawyers can definitely effect good social change. The problem, as Ashley pointed out, is that effecting positive social change doesn't pay very well, and so the vast bulk of students choose a well-paying BigLaw job over a low-paying Legal Aid or similar job, simply because they need to make their law school loan payments.
However, there are TONS of avenues through which you can be more than a "suit." As Ashley also rightly pointed out, almost all (if not all) law schools offer clinics where students can work with disadvantaged, indigent, and vulnerable populations (I like pointing out Harvard's, since they have tons:
http://www.law.harvard.edu/academics/clinical/clinics/index.html), and you can often extend the work that you do there into public interest work after graduation. There are also many internships and externships available while you're in school that can set you on that path.
My big thing, if you're really thinking about pursuing a career in public interest, would be to first focus on (a) applying to schools that have great fellowships/scholarships/grants for students with a public interest mindset, and (b) researching LRAPs (Loan Repayment Assistance Programs) at the schools you're interested in. LRAPs are programs that help students pay their law school loans if they go into a low-paying public interest/public service career. The better the LRAP, the more flexibility you will have. Typically, just searching "LRAP" on the school's website will bring up their information. I know George Washington Law in DC has a great one, as do Yale and Harvard. The ABA has a list of all the schools that have LRAPs, as well as state-funded LRAPs:
http://apps.americanbar.org/legalservices/probono/lawschools/pi_lrap.htmlFocus on figuring out how you'll pay for law school FIRST, so that you can relax and choose your career without having to make (too many) financial compromises.
I'm glad to give you more info on this, by the way. I'm VERY big on both public interest/public service, and thinking about financing law school BEFORE applying so that you can maximize your career choices.
