McDonald said the experience tested her nerves.
"They used to call you beforehand and ask if you accepted the role, but not any more. I knew that the announcement was coming, but had no idea who was getting what role.
"My stomach kind of dropped when my name got called out. It was really cool."
To be selected, McDonald first filled out an application form, which was followed by voting by Girls' College students on who they wanted to be their leaders.
Next came an interview process in which the candidates were drilled by assistant principal Craig Ritani and year 12 dean Caren Steed.
Finally, there was an overnight camp at the Jones St school grounds where all of the Year 13s participated in leadership and team-building activities.
McDonald recalled attending an assembly as a Year 9 in 2015 and said she was inspired by head girl at the time, Renee Harrison.
"She made herself really involved with everything. She put herself out there and got to know the students, which is what I'm aiming to do," McDonald said.
"I looked up to all the Year 13s. They'd sit on stage here and I always thought 'I want to be one of those people one day and be a role model to the other students'".
Helping McDonald will be the co-deputy heads of school, Emma Rennie and Sophie Kearney.
Five weeks into term one, the trio are already making moves.
One of McDonald's goals is to increase student input in the school and they have been able to start that process through the student council run by Kearney.
"We get to run assembly, so we wanted to try and make it more interesting for everyone else because everybody kind of dozes off while they're listening," McDonald said.
"We created an Instagram page which the students can follow and we put a survey on it asking what they would like more of in assembly. It worked really good, they asked us to play more videos so that they can interact with us."
McDonald said dancing had helped her build the skills that got her the gig as head of school.
She found ballet boring as a youngster and was informed of the Sharyn Underwood School of Dance by her friends Helena Hazelhurst and Lucy Dallas.
"I think dance really helped me with my leadership. When I was younger I liked taking control and doing dances with everyone," McDonald said.
"I've definitely become more confident and outgoing. I like to encourage lots of people to get involved, I'm energetic and I try to spread my vibes."
She enjoys hip-hop, jazz and contemporary and intends to make dancing her career.
In a couple of months, McDonald will audition for Ettingshausens, a dance and theatre, martial arts and career training school based in Sydney, Australia.
"I'm really hoping to get into that. It's a fulltime dance course, which is kind of scary, but I'm really excited," McDonald said.
"It's a really good school for making heaps of different contacts, so if companies ask where you've danced and you say Ettingshausens, that's very good.
"It would be amazing to dance on a show or with celebrity dancers and, although it might seem silly or unrealistic, that's a dream of mine."
