Can the company ask for declaration to work?

The agency that I am going to work for ask me for a written declaration to make a commitment for work during my University year are they eligible to ask for it? because i think they are not, I am signing the contract and I think that should be enough. And it's not fair because if I sign something like this I am not able to leave the work even if i just don't like the job or place or people working there, so for different reason than that the fact I might struggle to combine uni and work like they are thinking I might.