Looking at your flickr.. you definitely are good at portraiture. A lot better than I expected when clicking the link. So with that said..
You can shoot that 50 at 1.8. It's the big thing in senior portraiture right now, and it can be done without looking gimmicky, you just need to get that focus perfect. Make sure the focus is on their EYES, and the rest of the image will just follow. If the eyes are out of focus, it looks like shit.
On the topic of eyes, make sure you can see catch lights in them. People start to look dead when they don't have catch lights. Obviously not legitimately dead, but you know what I mean.
The picture you have of the girl on your flickr is really nice, it's back lit (very nice) and you still have catch lights. Perfect! The picture of the guy, that's exactly where you need to get catch lights. His eyes are small to begin with, so it's going to be harder to get light into them. But it's very very very very important. He looks like a zombie or something. It's a good shot, but you neeed to fix the eyes.
The biggest thing is look for nice lighting. A lot of people will tell you to shoot around sunset, like the few hours before it starts going down. You can, the light is nice, but if you want reallllyyy nice light, shoot at noon. Seriously. If you shoot mid-day, you can find open shadows (lighter shadows) and you can put the person in them. If a person is standing in the open shadow, the light will be reflecting off the surface infront of them that is getting direct sun, and it will act like a reflector and you will get those awesome catch lights that you're looking for. And using the ground rather than flashes or reflectors is the best. In my opinion.
Here's an example of a street at noon, apparently (I didn't take the picture), but see those shadows against the buildings? That's where you would shoot. That's the kind of shadow you're looking for.
Now on THAT note, GO LOCATION SCOUTING! For both the families and seniors, it's very important to have a few locations in mind that are relatively close to eachother so you can give them a variety of options. Also, it's really important you go at the time of day that you will be shooting, so that you know the light. Location scouting is fun, all you do is explore. And it will take some stress off the day of shooting that you already know the area.
If you can get your hands on other lenses, try out the 100mm Macro 2.8L. I use it for allll my portraiture (mostly senior portraits) and it's perfect. It's the sharpest, lightest, best lens EVER. But, don't go running out to buy it just for this, it is expensive. A good alternative you can buy is the 85mm 1.8. It's really nice, but with that 1.8 you NEED to make sure that the focus is perfect, otherwise your image is shit. But, once you get it right, it's beautiful.
And last but not least, check out photographers to get inspiration. It will help with everything, especially posing.
http://www.michellemooreblog.com/
Michelle Moore is one of the top senior portrait photographers in the country. She shoots almost always at noon, using back lighting and open shadows. She has a much looser, young, style. Copy her as much as you can, haha.
http://www.tarawhitney.com/
Tara Whitney shoots both seniors and families, but definitely check out her families, that's where she strives. Same deal as Michelle Moore, she shoots ALL natural light. Her family work is really candid and fun, but with some posed shots as well.
Hope that helps!
