Not happy about work

I like my job but I’m worried about fairness. I got a job in a school district so I recognize missing days is a challenge. I respect that and have yet to miss one. However, my coworker has missed constantly even when she first started the job. Doesn’t make the days up. Constantly claims her kids are sick. Issue is… when I finally need to take a day off for a doctor apt that is far away… I’m met with, you can make it up. Not to mention, when my coworker calls in, I have to do her work. I’m unsure if she’s receiving pay for it but I wouldn’t be surprised. I guess I’m feeling really frustrated about it.