Just teach ahead with the classes you meet. There will be many weeks ahead where there will be random 1 day vacations or there will be testing days. Just keep a diary and write which page/s each class does each lesson and continue to teach them from there. Some classes will be faster and others slower on certain topics too, so your classes will get out of sync. Don't stress the small stuff.
Oh, and that listening assessment, its a national assessment held only in the 3rd period (11am - 11:20 in actuality) each day, beginning with 1st Grade tomorrow, 2nd Grade Wednesday and 3rd Grade on Thursday. You will only miss 1 period each day. So it will be potentially 1 class a day IF you have a class scheduled in 3rd period.
So, from a seasoned teacher, just teach on.