Or you can wake up 15 minutes earlier and eat at home.

Public computer keyboards are among the most filthy, bacteria ridden suefaces in the school. You probably shouldn't be handling food and touching public keyboards at the same time.

And seriously, dealing with authority figures you don't agree with is a part of life. You will have teachers you don't like, but you'll have bad bosses, too. Learn how to adapt and suck up bullshit because life stops hand holding after high school where you start taking responsibility for yourself. 

Also, try having some empathy. Substitute teachers have a damn rough gig. Every day they get called into a class, they have to deal with a class they don't know who give them a hard time at best to being absolutely abusive more often than you'd think. So what if she questions you about eating in class? She wants to make sure you're not trying to get away with shit your normal teacher doesn't allow, which a lot of students do. Try getting a class of entitled, moody teenagers to pay attention or respect you when you're only there for a day or two.