I want to ask how can i not letting my school wifi know what i am doing in the computer?

The school's WiFi will be intended for school work. If you are doing something else, don't use their WiFi.

Wait until you get home to do that kind of computer things. Use the computer at school for homework like it is designed for.
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Maybe you could use Google and go incognito. That way there will be no way anyone can track you after you close ALL of the tabs.

They will know.But if you don't like this idea don't do anything illegal.

My school wifi blocks Facebook, Youtube, forums etc. Therefore, i use VPN to connect Facebook. Im afraid that my school can track it and block my VPN. What should i do now?