Lash a long broomstick to the lanyard of the flagpole. Put a garbage can on top of the stick. Raise the can to *ABOVE* the top of the flagpole, then let it back down. The can should catch the top of the pole, so you can retrieve the broomstick and stow the lanyard.
Now you have a garbage can on top of the flagpole.
Given the times & situation, it would be a much better thing to put the can on top of the flagpole that's flying the other school's flag, or state flag. Putting the can on top of the pole flying the US flag could be misinterpreted and get you in a sh*tload of trouble.
We did this many times over the course of two years (a couple decades ago), they never figured out how to get the can off without a "cherry picker" or platform-lift.
FWIW
Scott