My DD is 15 (not 16 until Aug). She was SA by her father and had no contact with him in 5 years. She has been going by my now married surname since after saying how seeing her previous surname all the time at school (fathers name) was negativity affecting her mental health. She has had her surname changed everywhere now and it has helped her.
She came back quite distraught from school after being told she had to use her legal surname on her GCSE papers. She can't change by deedpoll until she's 16 years old but due to her being one of the youngest in the year, that won't be until after she's sat her exams.
I'm not sure on how to help her. On on hand, it's just a name, but it clearly a big deal to her.