I was in high school and I missed my class for the entire semester.
Dreams about missing a class or being absent from school can often represent a feeling of unpreparedness or anxiety about a situation in waking life. It could be that you are feeling unprepared or overwhelmed about a project, task, or responsibility that you have. Alternatively, it could be a reflection of feeling like you've missed something in your life, or missed out on an opportunity. Your subconscious mind may be telling you that you need to get back on track and take responsibility for your actions in order to move forward. It may be helpful to reflect on what might be causing those feelings of anxiety or being unprepared in your waking life and take steps to address those issues.