What does it mean if my boss doing this?

I'm a 17 year old boy that got employed at a Jack in the Box (fast food) about a week ago. I applied to be a cashier, and yes I understand that cashiers also have to help with other tasks but hear me out. The actual manager at my job is a guy called Jose but he's out temporarily and left another manager called Liz in charge while he's gone. I went in last Tuesday for my first day. Liz asked if I was the kid Jose had hired and I said yes, but Liz wasn't told about me coming that day, so Liz just gave me a tour of the place and in the end she had me sweep, clean the tables, take out the trash, and clean the windows and told me to come again on Wednesday. Wednesday, I went in, and AGAIN Liz had me clean the whole time I was there but this time I was able to bag some orders and hand them to customers. I was told to come back again on Friday and that I'd get cashier training that day but once again on Friday Liz had me clean the whole time because she had forgotten the her supervisor would visit our location. At the end of Friday Liz told me to come back again on Wednesday (2 days from now). I haven't gotten trained as a cashier and Liz is just telling me days when she wants me to come instead of assigning me a schedule and I'm afraid of telling her I can't come in on certain days because of school because i don't want her to think I'm unreliable and possibly give me less days. What do you think I should do? Any advice would be appreciated. Thanks.