What Are Normal Iron Levels In Pre Teens?

My 141/2 grandaughter has just had a blood test done and her iron levels were minus-1 She is always cold irritable, tired and not a good eater always not feeling well and lacking concertration at school She claims she always feels sick and doesnt want to attend shool as she said she feels she cant concentrate. Also gets angry with family and friends regulary. Doctors say she isnt anemic but suggest Iron by intraveninance in hospital. What should her levels be and what is the cause and difference with low iron levels and anemia