hi
i am toddy a part of the kent group.
these are really safe as long as you wear the correct protection, the minium is wrist gaurds, i wear wrist and knee pads, some people wear helmets etc but it is all down to personal prefrence, and as to hurting your self, unless you start to do some of the very big tricks it is not very common.
as to boy's and girls, the admin( webmaster(naz)) and spud are both girls who bock, and there is quite a few in the kent group and in most groups there are some girls.
and as for trained coaches there is not any but most of the experienced bockers will and can teach new people well, to walk, run, bounce and then the tricks come later.
hope this helps, and i do teach people in the kent area if they want me to.