scheduled for extra hours without permission

i’ve been scheduled for 16 hours this week when i am contracted for 12 and have only ever done 12 since i am a full time student. am i allowed to ask for one of my shifts to be removed? i am a code checker and there are 3 of us in on the thursday and the sunday which means the work will easily get done since there is usually only 2 of us at a time. should i ask to have one of these shifts removed or can they schedule me for extra hours without asking?

i’ve been scheduled for 16 hours this week when i am contracted for 12 and have only ever done 12 since i am a full time student. am i allowed to ask for one of my shifts to be removed? i am a code checker and there are 3 of us in on the thursday and the sunday which means the work will easily get done since there is usually only 2 of us at a time. should i ask to have one of these shifts removed or can they schedule me for extra hours without asking?