I don't think we should practise on the mudbloods, they're not ours and do you really want to hack off Professor Carrow? Or even Madam Pince? I suppose Harry could practise on his mudblood but that wouldn't be fair to everyone else and I don't think he's going to loan Dennis out for hexing practise.

And absolutely not to practising on the house elves, don't you remember the tuna stuff they served for dinner that day that everyone complained about? Besides I'm not sure the hex even works on non-humans.