Hrm... looking through the thread on the "what ifs" part makes this a trickier one, but I'll see how I can do with those:
Case 1- She's clingy. You know these type, and it's also the most likely. "I can wait as long as it takes." "You're the only one for me." These are definitely the hardest type to shrug off. If they start doing this, drop the politeness, and add sternness to the voice tone. Something akin to: "I've already told you no. Even if you could wait, which a huge majority of people actually can't, I can't guarantee I'll even be the same person after at least 7 years. And no, I'm not the only one for you. If there truly was only 1 soulmate for every person on this earth, then why do so many people find one and get married? The odds for that are just too astronomical, and getting further out there every day. It must mean that there are multiple people which with they can get along. The same is true of you. Don't waste your time. Go find another guy; it's not going to happen with me."
Case 2- She's controlling. Technically easy to get rid of, it can be hard to do if it's someone who's a long-time friend. (Then again, a long-term friend shouldn't try to be controlling...) "It's not going to hurt you." "It'll be over before you know it." Things like this. You should know yourself best. (And he, himself, of course.) The way to get rid of these people is by taking control, and being somewhat rude in the process. Something like: "Listen. I know me better than you do. If you feel the need to start this relationship by taking control of me, it already failed. I am me, I'll be damned if I let you take away who I am. Now, drop it. It won't happen." Odds are pretty good that the controlling type will try to interject with 'but's, to which the only correct reply is 'no,' to be said RIGHT after she says 'but.' If she ignores your no and keeps talking, let her talk for a bit, but do not respond to anything she says. When there is a pause, jump back in with "Are you done talking? Because I wasn't. If you're not even willing to listen to me, then get out of my face. There's no chance in hell I'd spend the rest of my life taking orders from you." *Note: Sometimes, you get people who alternate between 1 and 2; switching between them as needed is acceptable.*
Case 3- The crying one. Unless preceded with a slap, most people will see this as a breakup; she is crying to get attention and to make him feel bad. Some guys are a sap for this; his naietivity means that this is a definite concern. The best thing to do here is to console them, trying to get them to look on the other side of the coin, though that will instead get most of them to go to a case 1 or 2, where you should follow those directions. I'd probably spin it like: "Don't cry. Look at it this way: Even if I were to be your boyfriend, I'd not be able to dedicate any time to you for at least 7 years. This is the same as you not having a BF for those 7 years; you'd have nobody to interact with on it. Instead, look for another guy. You're a nice person, it'll take you less than 7 years to find someone else, and then it's less time you spend alone. This really is for your own good and happiness." (Yes, I know that last sentence sounds like case 2; for some reason, it's more effective on the crying types.)
Case 4- The friend test. This is the easiest to get away from. Because a variety of questions will come up here, I can't give a sample dialogue. The way to get away from this is by looking at the med school future and a lack of time. The earlier paragraphs, before the case events, should help in this case.
Case 5- Asked flat-out. This one's just awkward, and will almost definitely be triggered by one question: "Are you gay?" (Nobody ever asks "Are you bisexual?", though it is the other question that will trigger this.) In addition to what you say, body language is suddenly very important here. (In the other cases, since there is no genuine interest, the body will be normal. The most common thing he'll need to be careful of is tensing up; someone who can read bodies will see this and know immediately that he's at least not straight.) While not strictly true, the best reaction I can think of would be a sort of disgusted confused face, followed by "What makes you ask that?" (Again with body reactions, an inquisitive tone is fine, an interested tone is not.)
From here, the questions, and thus, answers, become myriad. A few common cases are- 1. "Well, you don't have a girlfriend, and you're not interested in me.", 2. "I've heard a few people say you might be...", 3. "You've been seen hanging around with guys lately...", or 4. "Well, are you?" And corresponding answers would be: 1. "I don't have the time for one. Because I don't want to say I'll be with you and then neglect them, how does that make me gay?", 2. "And now you believe this? Why do you believe everything you hear?", 3. "What, so I can't be with my friends?" 4 will depend on which of the questions is asked. If it's the common one, he could truthfully say something like "No I'm not, but you're not my type either." If it's the other question, it'll take a bit more of a creative answer to avoid coming out. I might use "I do like women, but not your type."
I can't really give you a good escape route plan until I know the location. If it's her parent's place, for instance, walking out is incredibly rude, whereas it's not even an option if it's at his place. Lemme know that as soon as you know, and I'll see if I can't come up with a couple of those.