If everybody jumps in an airliner, would the pilots notice?
30000 lbs of people leaving and landing on the floors at the same time?  They'd notice something.  

Physically, the weight of the passengers on the floors would increase a bit as they pushed off and accelerated vertically.  Maybe 30000 lbs would increase to 35 or 40000 lbs for a split second, then once everybody's feet are clear, all the passengers are basically in a ballistic arc, so their weight is removed from the floors and the airplane is now 30000 lb lighter for a split second, then they would land and their vertical acceleration would apply maybe 40000 lbs to the floors for another split second, before returning to the steady state 30000 lbs.  So I would say it would feel to the flight crew like they hit a very small bump of turbulence, and there were be some kind of dull thud coming from behind from 300 feet landing at once.