At grammar school, I organised a very badly thought out and doomed to failure protest against school uniform rules, involving us all ripping down our hems and persuading every other girl to do so too. Followed by us all sewing them up again that same night. 
At college, I joined a demo demanding full student grants for all. Halfway through I realised I didn't even agree with it when a friend explained to a passerby that it wasn't fair that she didn't get a grant and I did, as her parents had more financial commitments than mine. These included the two cars, the big house in Cheshire and the holiday home in France. 
Recently I've been on the WASPI demos in London, which have all three clashed with other major events, but were no less well supported for all that (not given much media publicity though). I found them very cathartic.
The one I regret missing was the Iraq war protest, when I was out of the country.