Similar case happened in 3x years ago when I was in primary...
This situation happened on my elder brother who is two years older than me...
I think because our elder sister was too smart (she was ALWAYS be the first in her GRADE - not class, thru P1 to P6) that my brother had a lot of pressure... because we all were in the same school.
He was used to vomit in the morning after breakfast when we went to school... and he told me not to tell mother afterward.
Of course we all grow up now and were getting old now... but I always think that I should tell mother that time... My brother now has good health but not easily be getting fat.
Being a mother now, of course, my baby's health be in the first place rather than academic result.