It's generally not legal for a class full of students not to be under the supervision of a credentialed employee (teacher, counselor, administrator ...).
You'd need to research your own state's education code, however, for specifics about how such a situation should be handled.
Is it illegal for a school to not have a subsitute teacher assigned to a class when the teacher is not going to be in for the day and students just show up to class and no teacher apears for the whole class?
Is that legal?
2 answers
You might also check to see how substitute teachers are selected and notified. In our area, the system for notification is computerized and only three people are called. If they are not home or have valid excuses for not attending, then no substitute is assigned that day. Usually, if substitute teachers are no-shows without valid excuses, they are taken off the list.