When you're out sick from work, you should contact someone at work and let them know. Obviously if you're in the emergency room or something, this won't be the first thing on your mind; but if you're just feeling under the weather, you should really contact us to say you won't be in Thursday. Or Friday. Or Monday. Instead of showing up for your first day of work on Wednesday, and then vanishing from the face of the earth until next Tuesday, and claiming you didn't know who to contact.
Also, if you change your schedule for the day, and decide to come in for the afternoon shift instead of the morning as written on the schedule, you should let someone know.
Also, if you're supposed to come in at 10am, but you email to say you'd rather come in at 1pm, hey, points for contacting me! But if your reasoning is, you overslept and missed your 9am class, so you need to make it up with a noon class... that's not very impressive to your employer.