@satori00,
Having a stutter would not affect your concentration in class. However, it could be that whatever has caused your lack of concentration has exacerbated your stutter.
I would recommend seeing a doctor and maybe going to speech therapy.
You can never cure a stutter but you can manage it. The best way to do that is be up front about it, if you tell people you have a stutter it puts the pressure off and makes it easier to handle.
You need to think about what situations are more likely to make you stutter, for me it was in a one to one situation. I could stand on stage and address a crowd with no problems at all but talking on the phone or having a job interview was another matter entirely.
It does get better, but you really need to see a doctor about your lack of concentration.