I know what to do... What age are they? ... for every piece .. other than pawns unfortunately :/ make them do 10 push-ups, and if they are like under 10... have them do wall sits, trust me it works ! :D 

You could try giving them stress balls. Have them squeeze them whenever they can. This should help with grip. Giving them hard candy that change colors as layers are dissolved can be used as a quick fix until their grip gets better. They will take the candies out to compare and check on them giving them really sticky hands. Another trick that usually works is just sliding the pieces when you move. They have nowhere to fall if you drop them. I feel your pain. Picking up pieces can get so tedious if you have a lot students all at once. 

Dropping like "fall on the floor" or dropping like "what happens after it's hung"?

Magnetic sets are great.

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"Dropping like "fall on the floor" or dropping like "what happens after it's hung"?"

This is why real chess terms should be used, except when I am using them. En Prise solves the problem, well, not the problem of hanging pieces.

just get out the "whip". One sight of that, they're cured. :0)

Something which might work is to count the captures on the board every time.

This works for me pretty good. Every move I start systematically with this list: 1) look for ideas/patterns, 2) count the captures, 3) look for keyfields and only then I start calculating.

This minimized my blunders to almost zero.

The Heisman mantra of "checks, captures and threats" has helped me be much more consistent.  What are your opponent's checks, captures and threats at the beginning of each move, and what are yours?