Given two classes at once

Now, please don't misread the title. I'm not talking about having classes during conference periods. I'm not talking about having all gym classes at a high school or middle school. I'm talking about having two whole classes, for different subjects, at the same time, and I was NOT notified about this until the second class came in with all the rosters for this second class. I am peeved that they did not tell me this sooner. And I feel like I was set up for absolute disaster.

Edit to give details from a comment I made: 30 max students each class, all day, no plans for the second class at all, all high school, and while this first period is currently missing students due to testing, odds are rest of the day will be packed. Also 1st period is 2 hours long due to testing. And I am entirely by myself

UPDATE!: I was just told the teacher for Class 2 is back to get his class, so thankfully I'm down to the class I was originally assigned!