Three boys can effectively block a twisting staircase leading to the upstairs classrooms just as lessons resume after lunch. Congestion of M25 proportions ensues amidst chants of "goooo sloooowwww!" from all and sundry.

I used to do this and eventually escilated it slightly in the days before anyone had heard of road rage and the first of us got cars. On the way back from a couple of naughty splifs at lunch we would find it hilarious to drive at less than walking speed along the main hight street and see how many people we could enrage. Looking back not as clever as it seemed at the time.

Oh how we laughed. It was decided that Mr Baccus had to be delayed getting into maths. Maths was gay, y'see.
We paid a 1st year with 2 Marathons (not bloody "snickers") to lie, foetus-style, in the middle of the corridor during the lessons changes. The corridor had 4 rooms, for 4 classes. Duh. Roughly 100 children attempting to pass in this corridor whilst a small child lay groaning in the middle of it. Naturally, we aided the corridor congestion by loitering/throwing bags/throwing other first years into the melee. Mr Baccus did a poor job in keeping the children calm. Waving his arms around over his head a shouting "bloody children!" over and over did mot help his cause.
Not only did we stall Maths by a good 20 minutes, the whole school was reprimanded in assembly the next morning.
Break time however, saw dozens of our peers running around the playground/tennis courts imitating the frantic "Bloody children" pose and actions.

we had a similar door-jamming phenomenon where even me with my slight build could successfully hold back an entire class of lunch hungry kids (and teacher) for, ooh, seconds, by holding either side of the door frame tightly until the pressure got too much. I think the main reason I got away with it so lightly was that there were so many kids trying to get out that none of them had room to hit me...