pelos = por os
I am not 100% certain about the difference between 'para' and 'por', but here are my thoughts:
'para' indicates a direction or destination, so the students will receive these shirts
if you'd sell the shirts to raise money for a school trip, i'd say 'pelos' (por os) would be the better choice, it's used as 'for the sake of' / 'in order'
hope that's correct