So they know you have a stutter and still punish you or they just think you suck at speaking the language? In USA you can probably fight that..I'm not sure though I don't think it is officially labeled a disability under the ADA but lowering your scores because a stutter seems like something you could report at least. Have you straight up told your teacher 'FYI I have had a stutter my whole life and I apologize in advance if I fumble over some sounds'? I don't usually apologize for stuttering but that person seems like a douche so maybe try that?