You should arrive approximately 15 minutes before class starts.
For one-time classes, no admittance is allowed 15 minutes after the scheduled start of the class. This policy allows the teachers to have enough time to instruct and no have interruptions that affect the outcome for the entire class.
If you want to sit with friends at a one-time class, then someone must arrive early to save spaces. We do not save seats and if you arrive later you will have a hard time finding seats together.