On a pre-wedding shoot, or engagement shoot, this is as much for you as it is for them!

I use it as a tool to asses the couple and gauge their interaction... I always tell them if i'm going to pose them, I’ll let them know... Even some apparent candid shots are actually set up... Also letting them know will make them at ease rather than the couple feeling a bit blank as to what they are supposed to to, this could lead to making them feel a bit awkward. Something that instantly tells me how a couple are going to interact, is to get them to walk towards you in a nice setting, with about a 50 yard gap, tell them to walk towards you slowly, and just talk to each other, they will [i]usually[/i] hold hands, but i've had couples that haven't, and that tells me right away that i'm not going to get the posed up tight and all over each other!

Another great way to get them to show their real selves to you, is to stand them in front of each other, looking eye to eye, and tell them NOT to laugh... This will usually result in the couple in fits of laughter, and this really is their actual smile, and makes for great shots...

In all, keep the whole thing light and fun, and it will show through the images

As for how many shots should you take? I generally produce a range of about 20 images, but i've produced as low as 7 before (horrific weather), and the client be happy. How they are sold is another matter, depending what wedding package they have booked with me would depend on what they actually get.

Typically, a lower range package would mean they get a free 10x8 image and get an on-line gallery (I use zenfolio as the back end of my website, and this does protected hosting and point of sale) to purchase another prints they would like. A higher end package would get an on-line gallery and a CD/DVD of the images from the shoot in high resolution, not forgetting another folder of facebook optimised images with your logo on...

You should also remember to get them to sign a contract, even if it is between friends, this will also have details of model release so you can use the images later for promotional purposes!

Hope that's of some help!

My advice, has nothing to do with your questions, but this is what I know... (I haven't taken many pictures of people, and if I do, they would be candid shots.)
Always have extra charged camera batteries, if your camera needs special batteries then it's even more important to buy extra, because then it's not something you can just run to the nearest gas station for.
Tripods can be extra important if you have a finicky camera that blurs easily. (Remotes also help limit blur from shutter button movement, this is more essential for macro shots not farther focused shots).

I will try to answer your question about poses though...
Generally, fun shots, let people pose how they want, let them know that they look silly, but only if they are ok with silly and going for that kind of pose in the first place.
(Photographers feedback with people really helps. Also, I find that people can tell their pan-am smile from their natural smile, ask which one they prefer and let them go with that kind of smile. You should also bring a happy attitude obviously, people can tell when you've had a bad day, it can ware off.)
Formal shots/settings - these occasions are more set up, and are expected to be more professional, let them know what you're thinking and what they are doing, (not a lot of ongoing conversation obviously, but little comments that encourage the best in them - "bigger" "more subtle" "hold it" "good" ect…). Suggest to them the most successful poses that most people think is flattering. (Like the close up over the shoulder smile shot. The absent stare smile out a window.)
Candid - no advice, they are just candid. Lol (Always be mindful of the camera shy though of course. )
Simmi candid - those who are there and pretty and know you are taking pictures, but aren't posing like supermodels or something. Just letting you do your thing. These are the kind you can take a picture of a friend at a coffee shop.
Also, let them know your photography style if they don't already, know what styles you can pull off (lots of "styles" are dependent on post editing), if you feel uncomfortable/uncertain about a style, try taking a few practice shots and show them to see if they like it. Always be open to feedback, different people have different preferences within a style as well (different colors in post editing for instance).

I have a camera that can blur easily, and with no tripod or remote I generally take 30 or 40 pictures on one thing just so that I might get 5 good shots. (I have my camera set to continuous. The problem with continuous is that you can't use flash... at least not on my camera, and that sometimes, note, sometimes single shot mode is less blurry... but not always. From what I've seen, the cameras with the lenses you can add to the camera are better, because most every time, the pictures I see that are taken with the manual lenses are much more in focus than point and shoot cameras.)

So as for how many pictures you should take/ show them. Take as many as you can/want/think is needed. It depends on you and your style, and what the customer requests. Once when I was younger, my family had professional pictures taken of me (I would guess about 30 or 40) and the photographer took all his pictures and showed them to us allowing us to choose to preview between medium thumbnails and large thumbnails, and then he just clicked on the ones we showed interest in, then we chose the 3 or 4 we liked best.
As for nervousness... well I would start out with people I know first so that you can ease into the game and get some experience in, but if you're a head first kind of person then try to be confident, that might work better for you.
This is my two cents (I'm someone probably less experienced than you in this field so far. Kind of a hobbyist at the moment so far.)
I wish you the best blessings in your venture.

Two basic questions -

1) What did the school/prom committee ask for as far as type of photos required?

2) How big is the class?

Working a prom with a class of 20 students is very different than a class of 500.

If they are asking for just a wedding reception style of coverage (wander around and take candid photos) then, you might be able to get away with an online gallery where they can purchase prints.

If they are looking for a more traditional "mobile studio", you will need some form of organization. With a smaller class, you might be able to get away with something less formal - but with a large class, often you need a technique to match subject with prints and a way to distribute the prints. For large classes, many photographers work under a pre-order system (students pay for the photos at the time of shooting or as part of the prom tickets). Again, you could also go with an online gallery for these, but most photographers find that they make more sales when pre-ordering than off a gallery (many people look at the gallery, pick out the photos they plan on ordering and then never order).

Moved to the Photography forum as it is better suited here.