Sitting in seiza for 30 minutes is a piece of cake. i regularly attend meditation retreats where we sit up to 16 hours a day! I found that sitting in cross-legged meditation position got pretty agonizing after a couple of days so started alternating it with kneeling.
How long do you actually sit in seiza in class?In our dojo we may sit cross- legged during instruction, seiza is necessary only for bowing in and out. A couple of minutes shouldn't be harmful. For meditation you may want to use a bench but i recommend a zafu or meditation cushion, standing on edge to support your torso & keep the weight off your knees. Good luck.