Its unfortunately a fact of life that bullying goes on in all schools, primary, secondary [upper/comprehensive/grammar] Schools will often play it down, but it is there. [thruthfully it happens it in the work place too, we have to learn how to deal with it, which is of course easier for an adult]
Something I have mentioned before, the word grammar does not mean the school will be free from bullying.
My daughter, reasonably shy and bright originally wanted to go to a mixed grammar school, because it 'cut ' the girls down by half. That was either an intelligent or a sad statement to make, at the age of 10/11 she knew how 'bitchy' the girls could be.
She encountered many of these girls in her school life, but I taught her stratergies for dealing with said girls [one of which is now a very good friend!] On one occasion I rang the school, making it quite clear my child did not know I was making contact and I wanted it kept that way. The problem was solved diplomatically. She had a good circle of friends which have remained intact for some years.
One lesson in life is leaning how to react with different personalties and different situations.
I would leave for it now, but keep a mental note of the situation. However, has the bonding week finished ? Was it just the one remark or did it carry on in which case I would be tempted to make confidential contact.