It isn't only boys. I got bullied and obstructed as a kid by both girls AND boys, being one of the clumsy kids (and shy to boot - why is it that some people seem to hate this as a characteristic? Not everyone is an extrovert, and besides, who would listen to the extroverts, if everyone fell into that category?) As a kid it didn't help that I'm tiny and had no co-ordination as well as an eye condition. Oh, and a congenital dislike of team sports activities, particularly stupid games like hockey. I didn't mind team activities, just not sports where people could "accidentally" thump you and make your life a misery and get away with it, thanks. And apart from the swings, the playground was just like a team sport activity with no rules and no referees, in my experience.
On the "it's always boys, though", my younger sister has boys and I'm pretty sure she and her partner stopped them behaving like that when they were little, in fact I remember them as being very insistent that everyone who wanted to had a go in the paddling pool or on the Wii or whatever. They've grown up into fair-minded, kind adolescents.
My life improved somewhat when I was a bit older and could basically employ my innate sarcasm and extremely sharp tongue when necessary to deflect attention so I could get what I wanted or needed. Other friends used comedy and joking about to the same effect.
Basically, there are bullies everywhere. You see it all the time - on the motorway; in shops; on TV; in the playground, at Westminster. If you're not big enough to (metaphorically) thump them back, overpower them with your body language, or they just don't take any notice of you anyway, you need to find other ways to deal with them just to survive.
To be absolutely fair, the boys concerned here may not realise they were making it difficult for others. This is where mum needed to step in; good job this wasn't an accident.
Wouldn't it be wonderful if parenting skills, responsibility, the art of compromise, sharing, communication skills, and assertiveness, not aggression, were taught in schools instead of some of the rubbish they have to learn to pass exams?