She said her grandson has become "very quiet" since the attack but returned to school. The attackers, she said, appeared to be aged 15 or 16.
High school principal Karen Gilbert-Smith said the victims have been supported by senior staff while police investigate. The school does not know who the attackers are.
She would not say where the second attack happened.
"Obviously, it's a far from ideal way to start a year at school for the two young men involved but they have been well supported by their families and the school, and have counsellors available to them if they need further support."
Anyone who has information on the attackers is asked to call Whangarei police on (09) 470 4500 or Crimestoppers on 0800 555 111.