Why do they always get me to ride the worst horses?

At my riding school Im always given the horses that have an attitude or ones that get spooked easily. One horse named Gordon has been given to me every week for the past few months. He goes into the center of the arena even though i tried to get him to go the other way and i dont like using the whip often but im told to keep doing it. Gordon will try and buck me off him but my instructor just shouts at me to stay on. I do and eventually Gordon gives up and goes back into line. He's a big horse like a drought horse with furry feet except he's more slender cuase hes a cross breed. Im also given this horse that gets worked up and jittery when he has to get ready to canter and i have to walk him in a circle and then he just runs off and misses the jump hes supposed to go over. Im even given this huge black shire horse named Jack who hasan awful habit of biting and kicking other riders and horses and i have to wear high riding boots so he doestn bit my legs. Its very annoying and ive asked for a vreak and maybe the chance to rid a more docile horse but they refus and say im the only one who can handle the horses. Ive been riding since i was 3 (i used to ride shetlands with my dad beside me) and im 15 now but i still think im too young to be dealing with these horses. theyre huge guys and all muscular and bad tempered.