She needs to be seen by her doctor and have a throat swab done - a persistent sore throat, especially accompanied by a fever, could be strep throat (as well as the white spot on the throat). Left untreated, strept can lead to more serious complications.
Even if it's not strep, her symptoms definitely do point to an infection, which will require antibiotics to fully recover.
While I understand that it's not good for her to miss too much school, she really does need to be seen by a doctor and also, if she' has either strep or tonsillitis, she is contagious, so sending her to school is not only making her feel worse and perhaps extending her illness, but also exposing everyone else in the school to it.
Another thing to possibly ask the doc about checking her for would be mono. Mono can cause fever, sore throat and abdominal pain (sometimes due to an enlarged spleen) and severe fatigue. Again, mono is very contagious, so finding out if she has this is of utmost importance, not only for getting her treated and feeling better, but also so that anyone she's been around can also be tested and/or watch for symptoms.
I hope your niece feels better soon!