1st time posting, long-time reader (great info on this site).
This topic caught my attention, due to the prevalence of bat drag. I have found that bat drag, while difficult to see in person (why you should use video), produces the same outcomes depending on the pitcher:
Below average to average: Crushes the ball - hardly ever gets out (similar to BP)
Average: Sometimes runs into a ball - usually/always opposite field (unless he is catching it so far out in front that his wrist have started to roll and producing pull side 35 hoppers) - will start to K more often than not
Average to above average: K's will start to pile up and confidence will start to drop - Viscus cycle as the mental aspect of hitting is much more important than the mechanics (not saying mechanics are pointless, but I've seen a lot of "taught/robotic swings" produce little to nothing in clutch moments)
Elite: No chance in @#$! of touching the ball - ends up just hoping for a walk or not being the last out
SultanofSwat is right on point with this topic and his recommendations will help.
I've seen it be fixed with just a mirror. Kids need to figure out the way their body moves through the swing and most that have this problem can't show their swing to you in slow motion. They have yet to own their movements/swing.
Top hand drills work as well, but if they don't yet understand/feel the body chain of movements this can lead to a whole new set of problems. Throwing a ball to yourself in the air and hitting it to different parts of the field is also not a bad idea. Guy I used to know had his son do this a lot and he raked from the left side.
I take internet help with a grain of salt as most of it is detrimental, but Connor Powers (DeadRedHitting) has some good stuff. Shaft to shoulder drill worked well for my oldest son when he was younger and struggling with this issue. Coupled with studying Ted Williams (hips and hands) he turned his bat drag into lag in literally just a couple days. Helps that he has played just about every sport offered at one time or another, but the light-bulb went off when he started owning his swing.