A part of being a coach is you need to make sure you let the game be fun and competitive for the girls. This is because if you don't let the girls have fun, they mayget aggravated and do even worse. You don't want this to happen! As long as the team is doing everything correctly it shouldn't matter if they act serious or in a more fun state.
When teaching older girls, you usually don't have to do the basic volleyball drills. All this does it takes up a coach's time and it's pretty pointless at that. Being in high school, college, or club you should know the basics because usually you've played since you've been young. However, if you're teaching the young ones you need to make sure you do a lot of basic volleyball drills because that's the only way they'll get better. Depending on what age they're, you're only basic volleyball drills may be are serving and passing. When doing drills with younger kids you need to take it nice and slow with them and you can't be in a hurry. Also, a lot of coaches don't correct what the kids are doing wrong when they're young, but you really should, that's the only way they will learn.
However, since the younger kids need to stick with basic volleyball drills, the older girls can do things like volleyball blocking drills instead. Even if one group is young and the other one is older, you still can do creative volleyball drills for both, but they'll have a different difficultly to them.