Since there are 6 pairs, there are 12 team members, with T-shirts
numbered

1,2,3,4,5,6,7,8,9,10,11,12

Since each pair has the sum of the numbers on their T-shirts
the same, the 6 pairs are 

1 with 12, 2 with 11, 3 with 10,  4 with 9, 5 with 8, and 6 with 7

so that each pair has the sum of the numbers on their T-shirts as 13.

So the student with 5 on their T-shirt is paired with the student with
8 on their T-shirt.

Edwin