The service industry can be high turnover with people coming and going at all times. A restaurant manager asking this question wants to know if they can expect to keep you around for a while. Training new people is an investment on their part, and they don't want to waste time or money on someone just looking to make fast cash for a couple months of work. Let them know your plans, how this fits into your school schedule or how it balances with another job. Reassure them that you are looking for something long term.