Stuttering is unlike any other speech disorder and unlike most disorders in general. The fact that it's so fickle combined with the fact that its also different for each stutterer just adds to the plethora of misinformation. I'm sorry you have to deal with this. The grading thing is bullshit and if they're marking you down because of your stutter then I think you have a good complaint to the teacher, if not the principle. That's like marking down someone with cerebral palsy in gym class because they can't get a good mile time. And if people try to give you advice or tell you how to feel I would politely say, "thanks for the support but you don't have an idea what this is like". They can argue, but they even know if they were right.