There are two things you mentioned. Not able to concentrate is surely a serious issue, whether you are studying or working now. This is need of the hour and to be sorted out immediately. Was bullied by friends must be a painful incident that would have an impact as it would have left bitterness.  You can talk about this to your family members if it would work. If not, look for a Counsellor in your city and seek help.

Hi. Dont let friends bully you. You have to study well. Since your facing concentration focus issues you can meet a psychologist and get some concentration exercises.you should be more confident enough to handle your study timings.