Hello. I'm in a bit of a conundrum.

I'm working in an international school and recently the turnover rate is high so our company want us to sign a new contract that we may need to give 3-6 months notice.

All the staffs refused to sign it but then now they updated our staff guidebook stating this new rules about resignation notice. Our present contracts states that we abide to the staff guide book hence now my question is, does it means that whenever they update and change their guidebook, we have to follow it even though we didn't agree in the first place because the time we sign the contract , the guidebook does not have that rules.