i want to format my school mac

Do you own the MacBook Air? If so, you likely can reset the machine without going to an Apple Store. If this device is owned by the school and assigned to you, you should not attempt to reset the machine.

You do not provide the age of the device, but this article should send you on your way.

Erase your Mac and reset it to factory settings - Apple Support

Keep in mind, resetting the device will erase all data and 3rd party applications. Make sure you perform a backup before proceeding.

If your school owns the device, you may be able to wipe it and start again, but it will likely get the same policies and restrictions, and you could be breaking your acceptable use agreement.