I have found that for volleyball foot speed is essential. I stole most of these from the Core Performance workout. Its a great book for athletes. I would recommend purchasing that book or at least checking out his website.
Core Performance | Powered by Athletes' Performance
I usually do circuit training, for about 8 minutes, mostly because I do not want to waste too much practice time with fitness.
I do four stations, 1 minute at each station.
1. Lateral Bound (Starting with right foot on the ground and left in the air, jump as high while pushing off in a lateral motion. Land on your left foot, with knee left knee as close to 90 degrees as possible, once balanced on your left foot immediately jump high and laterally to your right, landing on your right foot with same knee bend so you are coiled and ready to jump again).
2. Squat (or block) Jumps - I prefer the squat jumps because they work more on balance and don't allow the girls to calf jump. Starting in a squatting position [weight mostly on your heels, knees as close to 90 degrees as possible] explode up as high as you can, land slightly ahead of where you took off in the same position you started, so you are coiled and ready to explode again.
3. Rapid Feet - Standing in an athletic position, knees slightly bent feet slight wider than shoulder width, have the kids jump over a line on the court. Think of this as a sprint. They need to move both feet over that line about back as fast as they possibly can. The moment their feet contact the ground they are pushing back to the other side of the line. With repetition this will increase foot speed.
4. One foot over line - same as above, but with one foot. Perform set/timed number then move to next foot. Again focus is speed, not height of jump.
5. Pushup to sprint - Do 10 pushups on the end line. On the tenth explode up using your arms, shoulder and back and sprint to the net. Jog back, high knees [exploding up off each leg, brining your knee to your chest]. Repeat for set amount of time.
6. Split squat lunge jump - Starting in a lunge position, right leg forward knees at 90 degrees. Explode up, jumping as high as you can from that position. Land in the exact same position, knees at 90 degree right leg still forward in lunge position. Once you land immediately explode back up. Do for set number of reps, or timed. You can challenge them back have their forward leg on an elevated object (stairs or something like that)
7. 3 hurdle drill - set up three hurdles each about 3 feet apart. starting with only your outside foot on the ground, hop the hurdle landing on your inside foot first followed by your outside foot, then hop over the middle hurdle with same progression. When you get to the 3rd hurdle only your outside foot hits the ground on the other side, while brining your inside foot up over the hurdle, but not setting it down. Push off with your outside foot and repeat going the opposite direction. Do for a set number of reps or timed. You can do different patters with this, be creative. Being fast and light on your feet are the keys here.
8. Rope Ladder - This one is great for foot speed. There are a number of different patters you can use and pretty much all of them are great. It will help increase foot speed.
9. Front and Side Planks or some variety of core specific exorcise.
I usually pair up two of these at each station and do 4 stations. I make the girls spend a minute at each station.
I hope you find some of this useful.