I understand that Polish uses other words when writing this sentence... but I sounds very odd in English to say "I am/am not carrying sandwiches" unless you are literally carrying them in your hands. In English it makes more sense to say I bring, or take.
No this sentence is very peculiar. It means exactly what is translated.
Where are you carrying those sanwiches? To school?
No, I am not carrying sandwiches to school.
Niosę is the "right now" kind of verb, along with idę, płynę etc.
Nie noszę/nie biorę/nie zabieram kanapek do szkoły are what would be translated I do not carry/take/bring sandwiches to school.
I do not know , because " i am not taking sandwiches to school" makes some sense and "Nie niosę kanapek do szkoły" does not.
That means I am taking/ I take sandwiches to school.
And nie biorę kanapek do szkoły means I am not taking/ I do not take sandwiches to school.