One difference is that your demo, I imagine, did not support a political party (given that all active political parties at that time were operating in the apartheid system, willingly or unwillingly.)
If you had dressed in ANC colours or waved the hammer and the sickle as part of your demo in the school I imagine you would have had more than a prickly principal to worry about...
I suppose it comes down to the difference in expressing views compared to political campaigning.
And, I do not know how old you are to know if you had the options these three lads have now, of publishing political campaigning in a school to the whole wide world.