Hello Brian, it is encouraging to know that there are young people out there who are concerned for the wellbeing of others. What your sister is experiencing is known as verbal harassment which is totally unacceptable.
This kind of harassment can be really upsetting and hurtful for the person involved, but also in other areas like in this case whereby your sister is considering to drop out of school. You can help her by encouraging her to report this to her class teacher who should be able to intervene. If this does not work she can talk to another trusted adult. Let her know that you support her and are there for her to talk to when things get tough. Please also encourage not to give up on her education.