You can try to go to some joint school activities.
There are many organisation that organise activities for all the secondary school student
Example of organisation : hksscu
Or you can try to go to some FB page and write a post anonymously, saying you want to find a girlfriend.
Example of Facebook page : 非名校secret, 中學生secret, 名校secret
P.S you can go to different schools' Christmas ball to meet new friends.
Many people find their girl/boyfriend there