You sir are a genius lol what do you think about using a small hula hoop?
Why not.The targt shouldn't be too big, but large enough so that players hit around 50% (maybe less in the first time he's going for target areas). just adjust the size.
You can use everything, be creative

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on hardcourts you an also use tape.
You could start very big, for example first devide the backcourt(beween service line and baseline) in two halfs by extending the line between the service boxes to the baseline. that sounds easy, but many players actually have a problem hitting that half of the backcourt ten times in a row, usually because they become shorter (balls that are shorter than to the service line are often killing players in matches).
to hit that area consistently in long rallies you need some placement skill. just hit the ball deep and into the right "box". As soon as you can do that consistently, make the box a little smaller by shifting the lines(i'm talking about the short and middle line-just use the side- and baseline for the outside lines) 4 feet behind the service line and 4 feet right or left of the middle. you will see that it becomes a little harder and you try until you master that.
If you do that progress regularly you will become a master in ball placement and run the opponent around the court in matches. makes much more sense than trying to shoot for a too small target.
this approach needs a little more patience, but is more satisfying and sucessfull in the end. in 2 or 3 months you will get real good sucess.
don't be shy to start real big. the kids will laugh first when you give them an entire half of the backcourt, but they will quickly stop laughing if they see that the struggle to fullfil the task of hitting that box ten times in a row. I have seen it taking 20 minutes till a team reaches the ten strokes per player (20 overall) even with solid junior players because they overhit, hit too short or too flat over the net.
This is also a thing to look for. pro strokes look flat, but usually they clear the net by 3-4 feet unless they are going for a winner. so span a line 3 feet over the net and let them hit nice,relaxed topsin shots over it. you will be surprised how much deeper and more consistent the players will hit.
too often young players try to "shave" the net to hit winners, which is not what the pros do:
http://www.youtube.com/watch?v=wTjBXVQyiwg
debt in strokes is 90% of the sucess, if you hit short you get chased around, but just by hitting deep topsin with some height you will beat most players even if you don't have a weapon.
If you coach a HS team that is not so good I would concentrate on very few things. do the same a lot. for example let them hit a lot of deep topsin shots over a line into the backcourt, mostly crosscourt. they will be bored, but if they can replicate those nice shots in a game and don't hit those short strokes into the service box they will be surprised how many matches they win without going for risky strokes. that doesn't mean just push, you can easily make pressure and run people around with those shots as nadal shows.
don't be too fine and try to do too many things. master a few things (for ex. the thing I mentioned) because if you do too many things they won't be able to translate that to matches.