But some schools are going a step further. They're organising other activities to promote this year's campaign theme "Go Beyond the Hour".
Rosaryhill School
Rosaryhill aims to raise students' awareness of the environment with debates.
One team from each of the school's six houses debates topics such as whether smoking should be banned in all public areas. The final round will be held today at lunchtime.
Teacher Kevin Leung Sung-yum, who organised the project, hopes the debates will give students a better understanding of environmental issues.
"Students can understand that there is more they can do apart from switching off the lights for one hour," he said.
The lights in all classrooms will also be switched off during lunchtime today. Teachers will spend 25 minutes asking students to think about what they are doing to protect the environment.
Five students were also chosen to take part in WWF's Earth Hour ambassador programme.
Using the skills their learned from WWF, the students have been spreading the green message at morning assemblies and in classroom talks.
Yuen Long Merchants Association Secondary School
Fifteen Form Six geography students at the school organised activities.
In one programme, they gave out about 70 seeds to their schoolmates to see whose seed could grow the fastest over several months.
They also made announcements over the school's public address system and gave out leaflets at lunchtime, encouraging students to flick the light switch.
They called for lights out in all classrooms during lunchtime today, and will present prizes for participation.
Canadian International School
The school's environment club launched a month-long campaign called "Throttle the Bottle" at the start of this month.
The project shows that sustainable living takes more than just turning off lights. To show how much plastic people use, club members have been collecting used plastic bottles from the recycling bins on each school floor.
At the end of each day, they wash the bottles and put them in a cage.
So far, there have been more than 1,500 bottles in the collection, which will be sent to a recycling company at the end of this month.
The club also organised a "no plastic bottle pledge" campaign. About 150 students and staff pledged not to use disposable plastic bottles at school. They all signed a huge banner to be displayed in the school's foyer.
In addition, the club organised a petition that was sent to the school's cafeteria, asking it not to sell plastic bottles of water anymore.
Next month, the students will hold their annual clean-up hike from Mui Wo to Pui O. Participants will pick up rubbish along beaches and the hiking trail.
YP junior reporter Debbie Wong says the campaigns encourage students to recycle and change their ideas about consumption.
Tsuen Wan Government Secondary School
The school set up a booth at its entrance on Wednesday and Thursday to test students' knowledge of environmental protection.
Those who answered a question correctly received an Earth Hour sticker or folder. Questions covered topics such as the greenhouse effect.
There will be a photo competition next month, with prizes for the best photos on the Earth Hour theme.
The school's nine Earth Hour student ambassadors will also distribute leaflets tomorrow morning in the school's neighbourhood. The lights of all classrooms will be switched off for an hour on Monday afternoon.
Additional reporting by Debbie Wong