First off, congrats on having call-backs, let alone at places you feel are "out of your league." Next, my two (or three) cents:
1. Any place that gives you a call-back is no longer out of your league. Such firms didn't become as big and prestigious as they are by accident - they know what they're doing, including their decision to spend/lose money by taking the time to interview you. (I had a call-back at, summered at, and will be returning as an associate to a firm which was, by any objective and reasonable measure, far out of my league. Once you're summering at such a firm, you will indeed notice that most of your fellow summers and pretty much all of the lawyers are very smart, with the finest educational credentials. Then you'll realize that you are actually completing your assignments to the lawyers' satisfaction and that you will receive an offer. Strange as it is and, yes, insofar as luck had anything to do with getting that summer position, you are now in "that league.")
2. Your classes be damned. You have call backs, so you're a serious (or just completely lucky) student, so your reluctance to miss classes is understandable. But your call-backs are the most important thing you have to do this semester (for that matter, they are the most important thing remaining in your legal education). You either (1) attend such a high ranked school that your professors expect students to miss lots of classes due to interviewing, or (2) attend a lower ranked school where your professors (or at least the administration) should be thanking you for having so many interviews (because it reflects well on them and the school). I would recommend scheduling call-backs on the earliest dates possible and at the earliest times available (i.e. morning, with a lunch). Most firms extend offers on a weekly basis, and as people accept, filling next summer's class on a rolling basis. The time during the day is really more my personal preference, but you should schedule your call-backs for sooner rather than later.
3. You need to banish from your mind the notion that interviewing means being "boring for 2.5 hours." Because they gave you a call-back, they know that you are smart enough (on paper) to score well on your first-year exams. The interview is your chance to confirm their judgment, i.e. it's when you show them that you are (1) smart, (2) friendly/personable, and (3) have a great attitude. These attributes correlate to one's ability to (1) learn quickly, (2) be liked by one's fellow attorneys when working endless hours together, and (3) work long, intense hours with a "please, give me more to do" attitude." The firm will look to confirm these same three attributes next summer, with the added attribute of "good judgment" also under review.
4. The call-back, like the job, is about stamina. You need to be just as smart, charming and enthusiastic during your last interview of the day as you were during the first. If you run into a dull or difficult interviewer, remain smart, pleasant and enthusiastic, and just get through that interview and get on to the next. Basically, when you get a call back (especially at the more prestigious firms), you already have the offer, but a "no" vote from a single lawyer will sink you. Again, the interview is just a means to confirm the firm's initial decision regarding your ability. Be smart, nice and enthusiastic with everyone, from the security gaurd and receptionist to the most junior associate and most senior partner. Also, if you go to lunch, it is usually with junior associates, many of whom will tell you that "you can relax now." Don't. Until you leave for the airport, you are in the interview, being scrutinized. (And try not to forget that next summer, once the booze starts flowing and junior associates really let their hair down.)