To be fair, I would have also assumed eating breakfast at the computer wasn't something you were supposed to be doing. We weren't allowed to bring anything food or drink related into the computer lab when I was in school. (Detention swiftly followed if you were caught with anything edible -- even something as innocuous as chewing gum.)

Suck it up, buttercup. It's a single period out of your day -- it's not worth getting into trouble over something so stupid as a sub you don't like. (The world will be full of people you hate that you have to get along with once you get out of school, so you might as well get used to dealing with it now. Such is life.)