Legaly can a company force you to pay back earned wages for terminating a contract.

I work as an intern for a large private manufacturing company in Wisconsin. When I started working for this company they made me sign a contract stating that I would stay with them until I earned my degree, and once I had my degree I would move into their supervisor training program to become a supervisor. The contract also stated that if I was to quit working with them before I stated the supervisor training program that I would have to pay back the company roughly a third of my earned wages. Also I do not know if this matters at all but I signed this contract with the facility I work in here in Wisconsin but I get paid from the corporate office in California and I don't think they know or have anything to do with the contract. At the time I needed a job so I signed the contract and have been working for them but I am now starting to look for other employers to try and get more experience, so I am wondering is this legal for them to do?