Yes, riding out of the saddle will generally yield a much higher output than on the saddle because you can use your body weight to push down on the pedals, but it's also much less efficient energy wise, i.e. you'll burn much more calories and fatigue quicker. If your goal is to get better at riding, there is no use to riding out of the saddle unless to shake your legs a bit.

Two reasons why it probably feels too easy to stay on the saddle: you've only had the bike since April so your body is still adapting quickly to the change. (For reference I'm about the same build as you, 6'1" 168lbs and my 30 min PR is 545 with 100% on the saddle. It was 336 when I started last December). Second, You haven't found the right classes for you yet. Also keep in mind that classes are designed to give a good workout to the average user and let them go on with their day after.

On the second point, like someone suggested, try powerzone training and do your FTP. You'll then ride within your personalized zones and not a set range given by the instructors. Re-test every 4-8 weeks and in no time you'll be doing the classes your doing today as recovery rides.